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Query-Meta Compatibility

This guideline will introduce how to investigate and manage the compatibility between databend-query and databend-meta.

Find out the versions​

  • To find out the build version of databend-query and its compatible databend-meta version:

    databend-query --cmd ver

    # output:
    version: 0.7.61-nightly
    min-compatible-metasrv-version: 0.7.59

    Which means this build of databend-query(0.7.61-nightly) can talk to a databend-meta of at least version 0.7.59, inclusive.

  • To find out the build version of databend-meta and its compatible databend-query version:

    databend-meta --cmd ver

    # output:
    version: 0.7.61-nightly
    min-compatible-client-version: 0.7.57

    Which means this build of databend-meta(0.7.61-nightly) can talk to a databend-query of at least version 0.7.57, inclusive.

Deploy compatible versions of databend-query and databend-meta​

A databend cluster has to be deployed with compatible versions of databend-query and databend-meta. A databend-query and databend-meta are compatible iff the following statements hold:

databend-query.version >= databend-meta.min-compatible-client-version
databend-bend.version  >= databend-query.min-compatible-metasrv-version
caution

If incompatible versions are deployed, an error InvalidArgument will occur when databend-query tries to connect to databend-meta, which can be found in databend-query log. Then databend-query will stop working.

How compatibility is checked​

Compatibility will be checked when a connection is established between meta-client(databend-query) and databend-meta, in a handshake RPC.

The client C(databend-query) and the server S(databend-meta) maintains two semantic-version:

  • C maintains the its own semver(C.ver) and the minimal compatible S semver(C.min_srv_ver).
  • S maintains the its own semver(S.ver) and the minimal compatible S semver(S.min_cli_ver).

When handshaking:

  • C sends its ver C.ver to S,
  • When S receives handshake request, S asserts that C.ver >= S.min_cli_ver.
  • Then S replies handshake-reply with its S.ver.
  • When C receives the reply, C asserts that S.ver >= C.min_srv_ver.

Handshake succeeds if both of these two assertions hold.

E.g.:

  • S: (ver=3, min_cli_ver=1) is compatible with C: (ver=3, min_srv_ver=2).
  • S: (ver=4, min_cli_ver=4) is NOT compatible with C: (ver=3, min_srv_ver=2). Because although S.ver(4) >= C.min_srv_ver(3) holds, but C.ver(3) >= S.min_cli_ver(4) does not hold.
C.ver:    1             3      4
C --------+-------------+------+------------>
          ^      .------'      ^
          |      |             |
          '-------------.      |
                 |      |      |
                 v      |      |
S ---------------+------+------+------------>
S.ver:           2      3      4