Move monzo_ynab into //tools

Optimizing is difficult: I like flat hierarchies because I don't like
directory-hopping, but I also would like a cleaner root for my mono-repo. Bombs
away!

Well it's that time again, folks: spring cleaning!

Here I am musing about a few things that bother me:
- Should I use kebab-case or snake_case?
- It feels ~confusing to have //tools and //utils. What a //projects? Isn't
  everything a project? *sigh*

15 files changed, 815 insertions, 0 deletions

diff --git a/tools/monzo_ynab/.envrc b/tools/monzo_ynab/.envrc
new file mode 100644
index 000000000000..14829cb12390
--- /dev/null
+++ b/tools/monzo_ynab/.envrc
@@ -0,0 +1,7 @@
+source_up
+export monzo_client_id="$(pass show finance/monzo/client-id)"
+export monzo_client_secret="$(pass show finance/monzo/client-secret)"
+export ynab_personal_access_token="$(pass show finance/youneedabudget.com/personal-access-token)"
+export ynab_account_id="$(pass show finance/youneedabudget.com/personal-access-token)"
+export ynab_budget_id="$(pass show finance/youneedabudget.com/budget-id)"
+export store_path="$(pwd)"
diff --git a/tools/monzo_ynab/.gitignore b/tools/monzo_ynab/.gitignore
new file mode 100644
index 000000000000..e92078303bec
--- /dev/null
+++ b/tools/monzo_ynab/.gitignore
@@ -0,0 +1,3 @@
+/ynab/fixture.json
+/monzo/fixture.json
+/kv.json
diff --git a/tools/monzo_ynab/README.md b/tools/monzo_ynab/README.md
new file mode 100644
index 000000000000..4ccbb35d8c5d
--- /dev/null
+++ b/tools/monzo_ynab/README.md
@@ -0,0 +1,41 @@
+# monzo_ynab
+
+Exporting Monzo transactions to my YouNeedABudget.com (i.e. YNAB) account. YNAB
+unfortunately doesn't currently offer an Monzo integration. As a workaround and
+a practical excuse to learn Go, I decided to write one myself.
+
+This job is going to run N times per 24 hours. Monzo offers webhooks for
+reacting to certain types of events. I don't expect I'll need realtime data for
+my YNAB integration. That may change, however, so it's worth noting.
+
+## Installation
+
+Like many other packages in this repository, `monzo_ynab` is packaged using
+Nix. To install and use, you have two options:
+
+You can install using `nix-build` and then run the resulting
+`./result/bin/monzo_ynab`.
+
+```shell
+> nix-build . && ./result/bin/monzo_ynab
+```
+
+Or you can install using `nix-env` if you'd like to create the `monzo_ynab`
+symlink.
+
+```shell
+> nix-env -f ~/briefcase/monzo_ynab -i
+```
+
+## Deployment
+
+While this project is currently not deployed, my plan is to host it on Google
+Cloud and run it as a Cloud Run application. What I don't yet know is whether or
+not this is feasible or a good idea. One complication that I foresee is that the
+OAuth 2.0 login flow requires a web browser until the access token and refresh
+tokens are acquired. I'm unsure how to workaround this at the moment.
+
+For more information about the general packaging and deployment strategies I'm
+currently using, refer to the [deployments][deploy] writeup.
+
+[deploy]: ../deploy/README.md
diff --git a/tools/monzo_ynab/auth.go b/tools/monzo_ynab/auth.go
new file mode 100644
index 000000000000..b66bacb10687
--- /dev/null
+++ b/tools/monzo_ynab/auth.go
@@ -0,0 +1,101 @@
+package auth
+
+////////////////////////////////////////////////////////////////////////////////
+// Dependencies
+////////////////////////////////////////////////////////////////////////////////
+
+import (
+	"encoding/json"
+	"fmt"
+	"log"
+	"net/http"
+	"net/url"
+	"os"
+	"os/exec"
+	"utils"
+)
+
+////////////////////////////////////////////////////////////////////////////////
+// Constants
+////////////////////////////////////////////////////////////////////////////////
+
+var (
+	BROWSER      = os.Getenv("BROWSER")
+	REDIRECT_URI = "http://localhost:8080/authorization-code"
+)
+
+////////////////////////////////////////////////////////////////////////////////
+// Types
+////////////////////////////////////////////////////////////////////////////////
+
+// This is the response returned from Monzo when we exchange our authorization
+// code for an access token. While Monzo returns additional fields, I'm only
+// interested in AccessToken and RefreshToken.
+type accessTokenResponse struct {
+	AccessToken  string `json:"access_token"`
+	RefreshToken string `json:"refresh_token"`
+	ExpiresIn    int    `json:"expires_in"`
+}
+
+type Tokens struct {
+	AccessToken  string
+	RefreshToken string
+	ExpiresIn    int
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Functions
+////////////////////////////////////////////////////////////////////////////////
+
+// Returns the access token and refresh tokens for the Monzo API.
+func GetTokensFromAuthCode(authCode string, clientID string, clientSecret string) *Tokens {
+	res, err := http.PostForm("https://api.monzo.com/oauth2/token", url.Values{
+		"grant_type":    {"authorization_code"},
+		"client_id":     {clientID},
+		"client_secret": {clientSecret},
+		"redirect_uri":  {REDIRECT_URI},
+		"code":          {authCode},
+	})
+	utils.FailOn(err)
+	defer res.Body.Close()
+	payload := &accessTokenResponse{}
+	json.NewDecoder(res.Body).Decode(payload)
+
+	return &Tokens{payload.AccessToken, payload.RefreshToken, payload.ExpiresIn}
+}
+
+// Open a web browser to allow the user to authorize this application. Return
+// the authorization code sent from Monzo.
+func GetAuthCode(clientID string) string {
+	// TODO(wpcarro): Consider generating a random string for the state when the
+	// application starts instead of hardcoding it here.
+	state := "xyz123"
+	url := fmt.Sprintf(
+		"https://auth.monzo.com/?client_id=%s&redirect_uri=%s&response_type=code&state=%s",
+		clientID, REDIRECT_URI, state)
+	exec.Command(BROWSER, url).Start()
+
+	authCode := make(chan string)
+	go func() {
+		log.Fatal(http.ListenAndServe(":8080",
+			http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+				// 1. Get authorization code from Monzo.
+				if req.URL.Path == "/authorization-code" {
+					params := req.URL.Query()
+					reqState := params["state"][0]
+					code := params["code"][0]
+
+					if reqState != state {
+						log.Fatalf("Value for state returned by Monzo does not equal our state. %s != %s", reqState, state)
+					}
+					authCode <- code
+
+					fmt.Fprintf(w, "Authorized!")
+				} else {
+					log.Printf("Unhandled request: %v\n", *req)
+				}
+			})))
+	}()
+	result := <-authCode
+	return result
+}
diff --git a/tools/monzo_ynab/dir-locals.nix b/tools/monzo_ynab/dir-locals.nix
new file mode 100644
index 000000000000..498f4b5055f8
--- /dev/null
+++ b/tools/monzo_ynab/dir-locals.nix
@@ -0,0 +1,3 @@
+let
+  briefcase = import <briefcase> {};
+in briefcase.utils.nixBufferFromShell ./shell.nix
diff --git a/tools/monzo_ynab/job.nix b/tools/monzo_ynab/job.nix
new file mode 100644
index 000000000000..1e10751012e2
--- /dev/null
+++ b/tools/monzo_ynab/job.nix
@@ -0,0 +1,12 @@
+{ depot, briefcase, ... }:
+
+depot.buildGo.program {
+  name = "job";
+  srcs = [
+    ./main.go
+  ];
+  deps = with briefcase.gopkgs; [
+    kv
+    utils
+  ];
+}
diff --git a/tools/monzo_ynab/main.go b/tools/monzo_ynab/main.go
new file mode 100644
index 000000000000..06f1944eab70
--- /dev/null
+++ b/tools/monzo_ynab/main.go
@@ -0,0 +1,43 @@
+// Exporting Monzo transactions to my YouNeedABudget.com (i.e. YNAB)
+// account. YNAB unfortunately doesn't currently offer an Monzo integration. As
+// a workaround and a practical excuse to learn Go, I decided to write one
+// myself.
+//
+// This job is going to run N times per 24 hours. Monzo offers webhooks for
+// reacting to certain types of events. I don't expect I'll need realtime data
+// for my YNAB integration. That may change, however, so it's worth noting.
+
+package main
+
+import (
+	"fmt"
+)
+
+var (
+	ynabAccountID = os.Getenv("ynab_account_id")
+)
+
+////////////////////////////////////////////////////////////////////////////////
+// Business Logic
+////////////////////////////////////////////////////////////////////////////////
+
+// Convert a Monzo transaction struct, `tx`, into a YNAB transaction struct.
+func toYnab(tx monzoSerde.Transaction) ynabSerde.Transaction {
+	return ynabSerde.Transaction{
+		Id: tx.Id,
+		Date: tx.Created,
+		Amount: tx.Amount,
+		Memo: tx.Notes,
+		AccountId: ynabAccountID,
+	}
+}
+
+func main() {
+	txs := monzo.TransactionsLast24Hours()
+	var ynabTxs []ynabSerde.Transaction{}
+	for tx := range txs {
+		append(ynabTxs, toYnab(tx))
+	}
+	ynab.PostTransactions(ynabTxs)
+	os.Exit(0)
+}
diff --git a/tools/monzo_ynab/monzo/client.go b/tools/monzo_ynab/monzo/client.go
new file mode 100644
index 000000000000..8c6c41e29f40
--- /dev/null
+++ b/tools/monzo_ynab/monzo/client.go
@@ -0,0 +1,52 @@
+package monzoClient
+
+import (
+	"fmt"
+	"log"
+	"monzoSerde"
+	"net/http"
+	"net/url"
+	"strings"
+	"time"
+	"tokens"
+	"utils"
+)
+
+const (
+	accountID = "pizza"
+)
+
+type Client struct{}
+
+// Ensure that the token server is running and return a new instance of a Client
+// struct.
+func Create() *Client {
+	tokens.StartServer()
+	time.Sleep(time.Second * 1)
+	return &Client{}
+}
+
+// Returns a slice of transactions from the last 24 hours.
+func (c *Client) Transactions24Hours() []monzoSerde.Transaction {
+	token := tokens.AccessToken()
+	form := url.Values{"account_id": {accountID}}
+	client := http.Client{}
+	req, _ := http.NewRequest("POST", "https://api.monzo.com/transactions",
+		strings.NewReader(form.Encode()))
+	req.Header.Add("Authorization", fmt.Sprintf("Bearer %s", token))
+	req.Header.Add("Content-Type", "application/x-www-form-urlencoded")
+	req.Header.Add("User-Agent", "monzo-ynab")
+	res, err := client.Do(req)
+
+	utils.DebugRequest(req)
+	utils.DebugResponse(res)
+
+	if err != nil {
+		utils.DebugRequest(req)
+		utils.DebugResponse(res)
+		log.Fatal(err)
+	}
+	defer res.Body.Close()
+
+	return []monzoSerde.Transaction{}
+}
diff --git a/tools/monzo_ynab/monzo/serde.go b/tools/monzo_ynab/monzo/serde.go
new file mode 100644
index 000000000000..a38585eca632
--- /dev/null
+++ b/tools/monzo_ynab/monzo/serde.go
@@ -0,0 +1,82 @@
+// This package hosts the serialization and deserialization logic for all of the
+// data types with which our application interacts from the Monzo API.
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"time"
+)
+
+type TxMetadata struct {
+	FasterPayment string `json:"faster_payment"`
+	FpsPaymentId  string `json:"fps_payment_id"`
+	Insertion     string `json:"insertion"`
+	Notes         string `json:"notes"`
+	Trn           string `json:"trn"`
+}
+
+type TxCounterparty struct {
+	AccountNumber string `json:"account_number"`
+	Name          string `json:"name"`
+	SortCode      string `json:"sort_code"`
+	UserId        string `json:"user_id"`
+}
+
+type Transaction struct {
+	Id                         string    `json:"id"`
+	Created                    time.Time `json:"created"`
+	Description                string    `json:"description"`
+	Amount                     int       `json:"amount"`
+	Currency                   string    `json:"currency"`
+	Notes                      string    `json:"notes"`
+	Metadata                   TxMetadata
+	AccountBalance             int            `json:"account_balance"`
+	International              interface{}    `json:"international"`
+	Category                   string         `json:"category"`
+	IsLoad                     bool           `json:"is_load"`
+	Settled                    time.Time      `json:"settled"`
+	LocalAmount                int            `json:"local_amount"`
+	LocalCurrency              string         `json:"local_currency"`
+	Updated                    time.Time      `json:"updated"`
+	AccountId                  string         `json:"account_id"`
+	UserId                     string         `json:"user_id"`
+	Counterparty               TxCounterparty `json:"counterparty"`
+	Scheme                     string         `json:"scheme"`
+	DedupeId                   string         `json:"dedupe_id"`
+	Originator                 bool           `json:"originator"`
+	IncludeInSpending          bool           `json:"include_in_spending"`
+	CanBeExcludedFromBreakdown bool           `json:"can_be_excluded_from_breakdown"`
+	CanBeMadeSubscription      bool           `json:"can_be_made_subscription"`
+	CanSplitTheBill            bool           `json:"can_split_the_bill"`
+	CanAddToTab                bool           `json:"can_add_to_tab"`
+	AmountIsPending            bool           `json:"amount_is_pending"`
+	// Fees interface{} `json:"fees"`
+	// Merchant interface `json:"merchant"`
+	// Labels interface{} `json:"labels"`
+	// Attachments interface{} `json:"attachments"`
+	// Categories interface{} `json:"categories"`
+}
+
+// Attempts to encode a Monzo transaction struct into a string.
+func serializeTx(tx *Transaction) (string, error) {
+	x, err := json.Marshal(tx)
+	return string(x), err
+}
+
+// Attempts to parse a string encoding a transaction presumably sent from a
+// Monzo server.
+func deserializeTx(x string) (*Transaction, error) {
+	target := &Transaction{}
+	err := json.Unmarshal([]byte(x), target)
+	return target, err
+}
+
+func main() {
+	b, _ := ioutil.ReadFile("./fixture.json")
+	tx := string(b)
+	target, _ := deserializeTx(tx)
+	out, _ := serializeTx(target)
+	fmt.Println(out)
+}
diff --git a/tools/monzo_ynab/requests.txt b/tools/monzo_ynab/requests.txt
new file mode 100644
index 000000000000..2da17c0b326a
--- /dev/null
+++ b/tools/monzo_ynab/requests.txt
@@ -0,0 +1,80 @@
+################################################################################
+# YNAB
+################################################################################
+:ynab = https://api.youneedabudget.com/v1
+:ynab-access-token := (getenv "ynab_personal_access_token")
+:ynab-budget-id := (getenv "ynab_budget_id")
+:ynab-account-id := (getenv "ynab_account_id")
+
+# Test
+GET :ynab/budgets
+Authorization: Bearer :ynab-access-token
+
+# List transactions
+GET :ynab/budgets/:ynab-budget-id/transactions
+Authorization: Bearer :ynab-access-token
+
+# Post transactions
+POST :ynab/budgets/:ynab-budget-id/transactions
+Authorization: Bearer :ynab-access-token
+Content-Type: application/json
+{
+  "transactions": [
+    {
+      "account_id": ":ynab-account-id",
+      "date": "2019-12-30",
+      "amount": 10000,
+      "payee_name": "Richard Stallman",
+      "memo": "Not so free software after all...",
+      "cleared": "cleared",
+      "approved": true,
+      "flag_color": "red",
+      "import_id": "xyz-123"
+    }
+  ]
+}
+
+################################################################################
+# Monzo
+################################################################################
+:monzo = https://api.monzo.com
+:monzo-access-token := (getenv "monzo_cached_access_token")
+:monzo-refresh-token := (getenv "monzo_cached_refresh_token")
+:monzo-client-id := (getenv "monzo_client_id")
+:monzo-client-secret := (getenv "monzo_client_secret")
+:monzo-account-id := (getenv "monzo_account_id")
+
+# List transactions
+GET :monzo/transactions
+Authorization: Bearer :monzo-access-token
+account_id==:monzo-account-id
+
+# Refresh access token
+# According from the docs, the access token expires in 6 hours.
+POST :monzo/oauth2/token
+Content-Type: application/x-www-form-urlencoded
+Authorization: Bearer :monzo-access-token
+grant_type=refresh_token&client_id=:monzo-client-id&client_secret=:monzo-client-secret&refresh_token=:monzo-refresh-token
+
+################################################################################
+# Tokens server
+################################################################################
+:tokens = http://localhost:4242
+
+# Get tokens
+GET :tokens/tokens
+
+# Get application state for debugging purposes
+GET :tokens/state
+
+# Force refresh tokens
+POST :tokens/refresh-tokens
+
+# Set tokens
+POST :tokens/set-tokens
+Content-Type: application/json
+{
+  "access_token": "access-token",
+  "refresh_token": "refresh-token",
+  "expires_in": 120
+}
diff --git a/tools/monzo_ynab/shell.nix b/tools/monzo_ynab/shell.nix
new file mode 100644
index 000000000000..aab4123bff9a
--- /dev/null
+++ b/tools/monzo_ynab/shell.nix
@@ -0,0 +1,9 @@
+let
+  pkgs = import <nixpkgs> {};
+in pkgs.mkShell {
+  buildInputs = [
+    pkgs.go
+    pkgs.goimports
+    pkgs.godef
+  ];
+}
diff --git a/tools/monzo_ynab/tokens.go b/tools/monzo_ynab/tokens.go
new file mode 100644
index 000000000000..4be967ccb803
--- /dev/null
+++ b/tools/monzo_ynab/tokens.go
@@ -0,0 +1,283 @@
+// Creating a Tokens server to manage my access and refresh tokens. Keeping this
+// as a separate server allows me to develop and use the access tokens without
+// going through client authorization.
+package main
+
+////////////////////////////////////////////////////////////////////////////////
+// Dependencies
+////////////////////////////////////////////////////////////////////////////////
+
+import (
+	"auth"
+	"encoding/json"
+	"fmt"
+	"io"
+	"kv"
+	"log"
+	"net/http"
+	"net/url"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+	"utils"
+)
+
+////////////////////////////////////////////////////////////////////////////////
+// Types
+////////////////////////////////////////////////////////////////////////////////
+
+// This is the response from Monzo's API after we request an access token
+// refresh.
+type refreshTokenResponse struct {
+	AccessToken  string `json:"access_token"`
+	RefreshToken string `json:"refresh_token"`
+	ClientId     string `json:"client_id"`
+	ExpiresIn    int    `json:"expires_in"`
+}
+
+// This is the shape of the request from clients wishing to set state of the
+// server.
+type setTokensRequest struct {
+	AccessToken  string `json:"access_token"`
+	RefreshToken string `json:"refresh_token"`
+	ExpiresIn    int    `json:"expires_in"`
+}
+
+// This is our application state.
+type state struct {
+	accessToken  string `json:"access_token"`
+	refreshToken string `json:"refresh_token"`
+}
+
+type readMsg struct {
+	sender chan state
+}
+
+type writeMsg struct {
+	state  state
+	sender chan bool
+}
+
+type channels struct {
+	reads  chan readMsg
+	writes chan writeMsg
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Top-level Definitions
+////////////////////////////////////////////////////////////////////////////////
+
+var chans = &channels{
+	reads:  make(chan readMsg),
+	writes: make(chan writeMsg),
+}
+
+var (
+	monzoClientId     = os.Getenv("monzo_client_id")
+	monzoClientSecret = os.Getenv("monzo_client_secret")
+	storePath         = os.Getenv("store_path")
+)
+
+////////////////////////////////////////////////////////////////////////////////
+// Utils
+////////////////////////////////////////////////////////////////////////////////
+
+// Print the access and refresh tokens for debugging.
+func logTokens(access string, refresh string) {
+	log.Printf("Access: %s\n", access)
+	log.Printf("Refresh: %s\n", refresh)
+}
+
+func (state *state) String() string {
+	return fmt.Sprintf("state{\n\taccessToken: \"%s\",\n\trefreshToken: \"%s\"\n}\n", state.accessToken, state.refreshToken)
+}
+
+// Schedule a token refresh for `expiresIn` seconds using the provided
+// `refreshToken`. This will update the application state with the access token
+// and schedule an additional token refresh for the newly acquired tokens.
+func scheduleTokenRefresh(expiresIn int, refreshToken string) {
+	duration := time.Second * time.Duration(expiresIn)
+	timestamp := time.Now().Local().Add(duration)
+	// TODO(wpcarro): Consider adding a more human readable version that will
+	// log the number of hours, minutes, etc. until the next refresh.
+	log.Printf("Scheduling token refresh for %v\n", timestamp)
+	time.Sleep(duration)
+	log.Println("Refreshing tokens now...")
+	accessToken, refreshToken := refreshTokens(refreshToken)
+	log.Println("Successfully refreshed tokens.")
+	logTokens(accessToken, refreshToken)
+	setState(accessToken, refreshToken)
+}
+
+// Exchange existing credentials for a new access token and `refreshToken`. Also
+// schedule the next refresh. This function returns the newly acquired access
+// token and refresh token.
+func refreshTokens(refreshToken string) (string, string) {
+	// TODO(wpcarro): Support retries with exponential backoff.
+	res, err := http.PostForm("https://api.monzo.com/oauth2/token", url.Values{
+		"grant_type":    {"refresh_token"},
+		"client_id":     {monzoClientId},
+		"client_secret": {monzoClientSecret},
+		"refresh_token": {refreshToken},
+	})
+	if res.StatusCode != http.StatusOK {
+		// TODO(wpcarro): Considering panicking here.
+		utils.DebugResponse(res)
+	}
+	if err != nil {
+		utils.DebugResponse(res)
+		log.Fatal("The request to Monzo to refresh our access token failed.", err)
+	}
+	defer res.Body.Close()
+	payload := &refreshTokenResponse{}
+	err = json.NewDecoder(res.Body).Decode(payload)
+	if err != nil {
+		log.Fatal("Could not decode the JSON response from Monzo.", err)
+	}
+
+	go scheduleTokenRefresh(payload.ExpiresIn, payload.RefreshToken)
+
+	// Interestingly, JSON decoding into the refreshTokenResponse can success
+	// even if the decoder doesn't populate any of the fields in the
+	// refreshTokenResponse struct. From what I read, it isn't possible to make
+	// these fields as required using an annotation, so this guard must suffice
+	// for now.
+	if payload.AccessToken == "" || payload.RefreshToken == "" {
+		log.Fatal("JSON parsed correctly but failed to populate token fields.")
+	}
+
+	return payload.AccessToken, payload.RefreshToken
+}
+
+func persistTokens(access string, refresh string) {
+	log.Println("Persisting tokens...")
+	kv.Set(storePath, "monzoAccessToken", access)
+	kv.Set(storePath, "monzoRefreshToken", refresh)
+	log.Println("Successfully persisted tokens.")
+}
+
+// Listen for SIGINT and SIGTERM signals. When received, persist the access and
+// refresh tokens and shutdown the server.
+func handleInterrupts() {
+	// Gracefully handle interruptions.
+	sigs := make(chan os.Signal, 1)
+	done := make(chan bool)
+
+	signal.Notify(sigs, syscall.SIGINT, syscall.SIGTERM)
+
+	go func() {
+		sig := <-sigs
+		log.Printf("Received signal to shutdown. %v\n", sig)
+		state := getState()
+		persistTokens(state.accessToken, state.refreshToken)
+		done <- true
+	}()
+
+	<-done
+	log.Println("Exiting...")
+	os.Exit(0)
+}
+
+// Set `accessToken` and `refreshToken` on application state.
+func setState(accessToken string, refreshToken string) {
+	msg := writeMsg{state{accessToken, refreshToken}, make(chan bool)}
+	chans.writes <- msg
+	<-msg.sender
+}
+
+// Return our application state.
+func getState() state {
+	msg := readMsg{make(chan state)}
+	chans.reads <- msg
+	return <-msg.sender
+}
+
+////////////////////////////////////////////////////////////////////////////////
+// Main
+////////////////////////////////////////////////////////////////////////////////
+
+func main() {
+	// Manage application state.
+	go func() {
+		state := &state{}
+		for {
+			select {
+			case msg := <-chans.reads:
+				log.Println("Reading from state...")
+				log.Println(state)
+				msg.sender <- *state
+			case msg := <-chans.writes:
+				log.Println("Writing to state.")
+				log.Printf("Old: %s\n", state)
+				*state = msg.state
+				log.Printf("New: %s\n", state)
+				// As an attempt to maintain consistency between application
+				// state and persisted state, everytime we write to the
+				// application state, we will write to the store.
+				persistTokens(state.accessToken, state.refreshToken)
+				msg.sender <- true
+			}
+		}
+	}()
+
+	// Retrieve cached tokens from store.
+	accessToken := fmt.Sprintf("%v", kv.Get(storePath, "monzoAccessToken"))
+	refreshToken := fmt.Sprintf("%v", kv.Get(storePath, "monzoRefreshToken"))
+
+	log.Println("Attempting to retrieve cached credentials...")
+	logTokens(accessToken, refreshToken)
+
+	if accessToken == "" || refreshToken == "" {
+		log.Println("Cached credentials are absent. Authorizing client...")
+		authCode := auth.GetAuthCode(monzoClientId)
+		tokens := auth.GetTokensFromAuthCode(authCode, monzoClientId, monzoClientSecret)
+		setState(tokens.AccessToken, tokens.RefreshToken)
+		go scheduleTokenRefresh(tokens.ExpiresIn, tokens.RefreshToken)
+	} else {
+		setState(accessToken, refreshToken)
+		// If we have tokens, they may be expiring soon. We don't know because
+		// we aren't storing the expiration timestamp in the state or in the
+		// store. Until we have that information, and to be safe, let's refresh
+		// the tokens.
+		go scheduleTokenRefresh(0, refreshToken)
+	}
+
+	// Gracefully handle shutdowns.
+	go handleInterrupts()
+
+	// Listen to inbound requests.
+	fmt.Println("Listening on http://localhost:4242 ...")
+	log.Fatal(http.ListenAndServe(":4242",
+		http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
+			if req.URL.Path == "/refresh-tokens" && req.Method == "POST" {
+				state := getState()
+				go scheduleTokenRefresh(0, state.refreshToken)
+				fmt.Fprintf(w, "Done.")
+			} else if req.URL.Path == "/set-tokens" && req.Method == "POST" {
+				// Parse
+				payload := &setTokensRequest{}
+				err := json.NewDecoder(req.Body).Decode(payload)
+				if err != nil {
+					log.Fatal("Could not decode the user's JSON request.", err)
+				}
+
+				// Update application state
+				setState(payload.AccessToken, payload.RefreshToken)
+
+				// Refresh tokens
+				go scheduleTokenRefresh(payload.ExpiresIn, payload.RefreshToken)
+
+				// Ack
+				fmt.Fprintf(w, "Done.")
+			} else if req.URL.Path == "/state" && req.Method == "GET" {
+				// TODO(wpcarro): Ensure that this returns serialized state.
+				w.Header().Set("Content-type", "application/json")
+				state := getState()
+				payload, _ := json.Marshal(state)
+				io.WriteString(w, string(payload))
+			} else {
+				log.Printf("Unhandled request: %v\n", *req)
+			}
+		})))
+}
diff --git a/tools/monzo_ynab/tokens.nix b/tools/monzo_ynab/tokens.nix
new file mode 100644
index 000000000000..97de09d741e9
--- /dev/null
+++ b/tools/monzo_ynab/tokens.nix
@@ -0,0 +1,23 @@
+{ depot, briefcase, ... }:
+
+let
+  auth = depot.buildGo.package {
+    name = "auth";
+    srcs = [
+      ./auth.go
+    ];
+    deps = with briefcase.gopkgs; [
+      utils
+    ];
+  };
+in depot.buildGo.program {
+  name = "token-server";
+  srcs = [
+    ./tokens.go
+  ];
+  deps = with briefcase.gopkgs; [
+    kv
+    utils
+    auth
+  ];
+}
diff --git a/tools/monzo_ynab/ynab/client.go b/tools/monzo_ynab/ynab/client.go
new file mode 100644
index 000000000000..0492b9071adc
--- /dev/null
+++ b/tools/monzo_ynab/ynab/client.go
@@ -0,0 +1,24 @@
+package client
+
+import (
+	"serde"
+)
+
+// See requests.txt for more details.
+func PostTransactions(accountID string, txs []serde.Transaction{}) error {
+	return map[string]string{
+		"transactions": [
+			{
+				"account_id": accountID,
+					"date": "2019-12-30",
+					"amount": 10000,
+					"payee_name": "Richard Stallman",
+					"memo": "Not so free software after all...",
+					"cleared": "cleared",
+					"approved": true,
+					"flag_color": "red",
+					"import_id": "xyz-123"
+			}
+		]
+	}
+}
diff --git a/tools/monzo_ynab/ynab/serde.go b/tools/monzo_ynab/ynab/serde.go
new file mode 100644
index 000000000000..53dd33e83637
--- /dev/null
+++ b/tools/monzo_ynab/ynab/serde.go
@@ -0,0 +1,52 @@
+// This package hosts the serialization and deserialization logic for all of the
+// data types with which our application interacts from the YNAB API.
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"time"
+)
+
+type Transaction struct {
+	Id           string    `json:"id"`
+	Date         time.Time `json:"date"`
+	Amount       int       `json:"amount"`
+	Memo         string    `json:"memo"`
+	Cleared      string    `json:"cleared"`
+	Approved     bool      `json:"approved"`
+	FlagColor    string    `json:"flag_color"`
+	AccountId    string    `json:"account_id"`
+	AccountName  string    `json:"account_name"`
+	PayeeId      string    `json:"payeed_id"`
+	PayeeName    string    `json:"payee_name"`
+	CategoryId   string    `json:"category_id"`
+	CategoryName string    `json:"category_name"`
+	Deleted      bool      `json:"deleted"`
+	// TransferAccountId interface{} `json:"transfer_account_id"`
+	// TransferTransactionId interface{} `json:"transfer_transaction_id"`
+	// MatchedTransactionId interface{} `json:"matched_transaction_id"`
+	// ImportId interface{} `json:"import_id"`
+	// Subtransactions interface{} `json:"subtransactions"`
+}
+
+// Attempts to encode a YNAB transaction into a string.
+func serializeTx(tx *Transaction) (string, error) {
+	x, err := json.Marshal(tx)
+	return string(x), err
+}
+
+// Attempts to parse a string encoding a transaction presumably sent from a
+// YNAB server.
+func deserializeTx(x string) (*Transaction, error) {
+	target := &Transaction{}
+	err := json.Unmarshal([]byte(x), target)
+	return target, err
+}
+
+func main() {
+	target, _ := deserializeTx(tx)
+	out, _ := serializeTx(target)
+	fmt.Println(out)
+	fmt.Println(ynabOut)
+}