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+# Logging
+
+Because the daemon protocol only has one sender stream and one receiver stream
+logging messages need to be carefully interleaved with requests and responses.
+Usually this means that after the operation and all of its inputs (the request)
+has been read logging hijacks the sender stream (in the server case) and uses
+it to send typed logging messages while the request is being processed. When
+the response has been generated it will send `STDERR_LAST` to mark that what
+follows is the response data to the request. If the request failed a
+`STDERR_ERROR` message is sent with the error and no response is sent.
+
+While not in this state between request reading and response sending all
+messages and activities are buffered until next time the logger can send data.
+
+The logging messages supported are:
+- `STDERR_LAST`
+- `STDERR_ERROR`
+- `STDERR_NEXT`
+- `STDERR_READ`
+- `STDERR_WRITE`
+- `STDERR_START_ACTIVITY`
+- `STDERR_STOP_ACTIVITY`
+- `STDERR_RESULT`
+
+
+### `STDERR_LAST`
+Marks the end of the logs, normal processing can resume.
+
+- 0x616c7473 :: [UInt64][se-UInt64] (hardcoded)
+
+### `STDERR_ERROR`
+This also marks the end of this log "session" and so it
+has the same effect as `STDERR_LAST`.
+On the client the error is thrown as an exception and no response is read.
+
+#### If protocol version is 1.26 or newer
+- 0x63787470 :: [UInt64][se-UInt64] (hardcoded)
+- error :: [Error][se-Error]
+
+#### If protocol version is older than 1.26
+- 0x63787470 :: [UInt64][se-UInt64] (hardcoded)
+- msg :: [String][se-String]
+- exitStatus :: [Int][se-Int]
+
+
+### `STDERR_NEXT`
+Normal string log message.
+
+- 0x6f6c6d67 :: [UInt64][se-UInt64] (hardcoded)
+- msg :: [String][se-String]
+
+
+### `STDERR_READ`
+Reader interface used by ImportsPaths and AddToStoreNar (between 1.21 and 1.23).
+It works by sending a desired buffer length and then on the receiver stream it
+reads bytes buffer of that length. If it receives 0 bytes it sees this as an
+unexpected EOF.
+
+- 0x64617461 :: [UInt64][se-UInt64] (hardcoded)
+- desiredLen :: [Size][se-Size]
+
+### `STDERR_WRITE`
+Writer interface used by ExportPath. Simply writes a buffer.
+
+- 0x64617416 :: [UInt64][se-UInt64] (hardcoded)
+- buffer :: [Bytes][se-Bytes]
+
+### `STDERR_START_ACTIVITY`
+Begins an activity. In other tracing frameworks this would be called a span.
+
+Implemented in protocol 1.20. To achieve backwards compatible with older
+versions of the protocol instead of sending an `STDERR_START_ACTIVITY`
+the level is checked against enabled logging level and the text field is
+sent as a simple log message with `STDERR_NEXT`.
+
+- 0x53545254 :: [UInt64][se-UInt64] (hardcoded)
+- act :: [UInt64][se-UInt64]
+- level :: [Verbosity][se-Verbosity]
+- type :: [ActivityType][se-ActivityType]
+- text :: [String][se-String]
+- fields :: [List][se-List] of [Field][se-Field]
+- parent :: [UInt64][se-UInt64]
+
+
+act is atomic (nextId++ + (getPid() << 32))
+
+
+### `STDERR_STOP_ACTIVITY`
+Stops the given activity. The activity id should not send any more results.
+Just sends `ActivityId`.
+
+Implemented in protocol 1.20. When backwards compatible with older versions of
+the protocol and this message would have been sent it is instead ignored.
+
+- 0x53544f50 :: [UInt64][se-UInt64] (hardcoded)
+
+
+### `STDERR_RESULT`
+Sends results for a given activity.
+
+Implemented in protocol 1.20. When backwards compatible with older versions of
+the protocol and this message would have been sent it is instead ignored.
+
+- 0x52534c54 :: [UInt64][se-UInt64] (hardcoded)
+- act :: [UInt64][se-UInt64]
+- type :: [ResultType][se-ResultType]
+- fields :: [List][se-List] of [Field][se-Field]
+
+
+
+
+[se-UInt64]: ./serialization.md#uint64
+[se-Int]: ./serialization.md#int
+[se-Size]: ./serialization.md#size
+[se-Verbosity]: ./serialization.md#verbosity
+[se-ActivityType]: ./serialization.md#activitytype
+[se-ResultType]: ./serialization.md#resulttype
+[se-Bytes]: ./serialization.md#bytes
+[se-String]: ./serialization.md#string
+[se-List]: ./serialization.md#list-of-x
+[se-Error]: ./serialization.md#error
+[se-Field]: ./serialization.md#field
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