Fix: Fail early when database server does not respond #711
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""" WalkthroughThe changes update the connection logic to fail early when the database server is unresponsive, by raising the last encountered connection error if no valid server version is found. The changelog is updated to reflect this. Additionally, a new test is introduced to verify that connection errors are properly raised for invalid server addresses. Changes
Sequence Diagram(s)sequenceDiagram
participant Client
participant Connection
participant Server
Client->>Connection: connect()
loop For each server
Connection->>Server: request server version
alt Server responds with version
Connection->>Connection: store version if valid
else Server not available (ConnectionError)
Connection->>Connection: store last ConnectionError
else Invalid version (ValueError/InvalidVersion)
Connection->>Connection: ignore and continue
end
end
alt No valid server version found and ConnectionError occurred
Connection-->>Client: raise last ConnectionError
else Valid server version found
Connection-->>Client: proceed with connection
end
Poem
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Problem
@shraik started using sqlalchemy-cratedb and reported that its behaviour deviates from other vendors by not failing on
engine.connect()when the database server is not available.We found this is not actually on the SQLAlchemy dialect, but on the DBAPI driver already, which exhibits the same behaviour.
Solution
The patch extends the
_lowest_server_versionmethod to re-raise the lastConnectionErrorwhen no connection can be made to any configured server node. By doing it this way, we didn't need to submit a dummy SQL command like planned. It is much better this way because it does not pollute the server-side statement log./cc @surister