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Compatibility

This guideline will introduce how to investigate and manage the compatibility:

  • between databend-query and databend-meta.
  • between different versions of databend-meta.

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-versions:

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

Compatibility status​

The following is an illustration of current query-meta compatibility:

Meta\Query[0.7.59, 0.8.80)[0.8.80, 0.9.41)[0.9.41, 1.1.34)[1.1.34, +∞)
[0.8.30, 0.8.35)✅❌❌❌
[0.8.35, 0.9.23)✅✅✅❌
[0.9.23, 0.9.42)❌✅✅❌
[0.9.42, 1.1.32)❌❌✅❌
[1.1.32, +∞)❌❌✅✅

Compatibility of Databend-Meta On-Disk Data

The on-disk data of Databend-meta evolves over time while maintaining backward compatibility.

Identifying the Versions​

Upon startup, Databend-meta will display the on-disk data version:

For example, running databend-meta --single produces:

Databend Metasrv

Version: v1.1.33-nightly-...
Working DataVersion: V0

On Disk Data:
    Dir: ./.databend/meta
    Version: version=V0, upgrading=None
  • Working DataVersion denotes the version Databend-meta operates on.
  • On Disk Data -- DataVersion denotes the version of the on-disk data.

The Working DataVersion must be greater than or equal to the on-disk DataVersion; otherwise, it will panic.

The on-disk DataVersion must be compatible with the current Databend-meta version. If not, the system will prompt the user to downgrade Databend-meta and quit with a panic.

Automatic upgrade​

When databend-meta starting up, the on-disk is upgraded if it is compatible with the working DataVersion. The upgrade progress will be printed to stderr and to log file at INFO level, e.g.:

Upgrade on-disk data
    From: V0(2023-04-21: compatible with openraft v07 and v08, using openraft::compat)
    To:   V001(2023-05-15: Get rid of compat, use only openraft v08 data types)
Begin upgrading: version: V0, upgrading: V001
Write header: version: V0, upgrading: V001
Upgraded 167 records
Finished upgrading: version: V001, upgrading: None
Write header: version: V001, upgrading: None

If databend-meta crashes before upgrading finishes, it will clear partially upgraded data and resume the upgrade when it starts up again.

Backup data​

  • The exported backup data can only be imported with the same version of databend-metactl.

  • The first line of the backup is the version, e.g.: ["header",{"DataHeader":{"key":"header","value":{"version":"V100","upgrading":null}}}]

  • NO automatic upgrade will be done when importing. Automatic upgrade will only be done when databend-meta is brought up.