Anatomy of a Write Operation

Now we can trace how GFS handles a write. It needs more coordination than a read does.

What is a chunk lease?

GFS uses a lease, a time-bound grant of authority over one chunk. It keeps two replicas from taking conflicting writes at the same time. A mutation is any operation that changes a chunk: a write, an append, or a delete. When a client asks for one, the master finds the ChunkServers holding that chunk. It grants a lease, good for 60 seconds, to one of them.

That server becomes the primary. Its job is to put the chunk's pending mutations in a single serial order