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  1. IMPALA
  2. IMPALA-5476

Catalogd restart bring about metadata is out of sync

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      I having been using impala in our env. Here is our cluster deployment:
      20+ impalad backend.
      4 of all impalads act as coordinator.
      one catalogd and one statestored

      I encounter one problem that one impalad's metadata is out of sync after catalogd restart.I find that while catalogd restarting, a DML operation is executing.
      After I analyze impala source code, I reappear the problem. this is my steps and analysis:

      1. Start the impala cluster.
      2. The cluster run a long time, lots of metadata operations, and current catalogVersion_ is big(such as bigger than 10000)
      3. Submit a DML query(such as 'insert into xx partition() select xxx') to one impalad, and the query run about 1m.
      4. While the query running, I stop catalogd, and I start catalogd just before the query execute QueryExecState->UpdateCatalog().
      5. UpdateCatalog() will request catalogd for UpdateCatalog and catalogd will update the metadata of the table and response the newest metadata of the table.
      6. After catalogd response, UpdateCatalog() update metadata cached in impalad(call updateCatalogCache()), and the run the following code:

      if (!catalogServiceId_.equals(req.getCatalog_service_id())) {
      boolean firstRun = catalogServiceId_.equals(INITIAL_CATALOG_SERVICE_ID);
      catalogServiceId_ = req.getCatalog_service_id();
      if (!firstRun)

      { // Throw an exception which will trigger a full topic update request. throw new CatalogException("Detected catalog service ID change. Aborting " + "updateCatalog()"); }

      }

      serviceId is the new started catalogd's serviceId and do not equals to the impalad's catalogServiceId_, so the function throw CatalogException and the query get EXCEPTION, what is more, the impalad's catalogServiceId_ is set to the new one.

      7. After catalogd start successfully, and publish all metadata to statestored, then push to the impalad, After step 6, impalad's catalogServiceId_ equals to the catalogd's serviceId, no exception throws.

      8. In normal steps, step 7 will throw the CatalogException and set the from_version to 0 and statestored send full metadatas to impalad in next UpdateState().

      9. After all steps finish, the impalad is out of sync, all new metadata operation will be lost because CatalogObjectCache.add() need 'new item will only be added if it has a larger catalog version'.

      I said "new metadata will be lost" means the following metadata operation which happened to the existing table will be lost until the table's version catch up with the older version. I think any operation can not recover it because impalad update local cached metadata by comparing new version and older version.

      I try some operations which can trigger table's metadata reloading and new version generated such as refresh, alter table. But new metadata always lost until the catalog_version bigger than the older one, for new created catalog object(such as create table/ create database..), the metadata is up-to-date.

      I think it is a bug, we need keep older catalogServiceId_ until full newly metadata applied(non-delta one, pushed by statestored), even all of metadata operations will be EXCEPTION at this time gap. perhaps there are some better solutions.

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            liuyao liuyao
            feng_xiao_yu fengYu
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