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connection-state.md

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Connection state

There are some function to track the connection state. In Postgres, the state of a connection might be one of these:

  • idle when it's ready for a query;

  • in transaction when a transaction has already been started but not committed yet;

  • error when transaction has failed but hasn't been rolled back yet.

The status function returns either :I, :T, or :E keywords depending on where you are at the moment. For each state, there is a corresponding predicate that returns either true of false.

At the beginning, the connection is idle:

(pg/status conn)
:I

(pg/idle? conn)
true

Open a transaction and check out the state:

(pg/begin conn)

(pg/status conn)
:T

(pg/in-transaction? conn)
true

Now send a broken query to the server. You'll get an exception describing the error occurred on the server:

(pg/query conn "selekt dunno")

;; Execution error (PGErrorResponse) at org.pg.Accum/maybeThrowError (Accum.java:205).
;; Server error response: {severity=ERROR, code=42601, ...,  message=syntax error at or near "selekt", verbosity=ERROR}

The connection is in the error state now:

(pg/status conn)
:E

(pg/tx-error? conn)
true

When state is error, the connection doesn't accept any new queries. Each of them will be rejected with a message saying that the connection is in the error state. To recover from an error, rollback the current transaction:

(pg/rollback conn)

(pg/idle? conn)
true

Now it's ready for new queries again.