Secondary NameNode check-pointing process


Secondary namenode, whose purpose is to produce checkpoints of the primary’s in-memory filesystem metadata.

The check pointing process proceeds as follows,
1. The secondary asks the primary to roll its edits file, so new edits go to a new file.
2. The secondary retrieves fsimage and edits from the primary (using HTTP GET).
3. The secondary loads fsimage into memory, applies each operation from edits, then creates a new consolidated fsimage file.
4. The secondary sends the new fsimage back to the primary (using HTTP POST).
5. The primary replaces the old fsimage with the new one from the secondary, and the old edits file with the new one it started in step 1. It also updates the fstime file to record the time that the checkpoint was taken.

At the end of the process, the primary has an up-to-date fsimage file and a shorter edits file (it is not necessarily empty, as it may have received some edits while the checkpoint was being taken). It is possible for an administrator to run this process manually while the namenode is in safe mode, using the hadoop dfsadmin

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