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118 lines
3.3 KiB
118 lines
3.3 KiB
Distributed Filer
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===========================
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The default weed filer is in standalone mode, storing file metadata on disk.
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It is quite efficient to go through deep directory path and can handle
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millions of files.
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However, no SPOF is a must-have requirement for many projects.
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Luckily, SeaweedFS is so flexible that we can use a completely different way
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to manage file metadata.
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This distributed filer uses Redis or Cassandra to store the metadata.
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Redis Setup
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#####################
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No setup required.
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Cassandra Setup
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#####################
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Here is the CQL to create the table.CassandraStore.
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Optionally you can adjust the keyspace name and replication settings.
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For production, you would want to set replication_factor to 3
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if there are at least 3 Cassandra servers.
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.. code-block:: bash
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create keyspace seaweed WITH replication = {
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'class':'SimpleStrategy',
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'replication_factor':1
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};
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use seaweed;
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CREATE TABLE seaweed_files (
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path varchar,
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fids list<varchar>,
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PRIMARY KEY (path)
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);
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Sample usage
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#####################
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To start a weed filer in distributed mode with Redis:
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.. code-block:: bash
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# assuming you already started weed master and weed volume
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weed filer -redis.server=localhost:6379
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To start a weed filer in distributed mode with Cassandra:
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.. code-block:: bash
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# assuming you already started weed master and weed volume
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weed filer -cassandra.server=localhost
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Now you can add/delete files
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.. code-block:: bash
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# POST a file and read it back
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curl -F "filename=@README.md" "http://localhost:8888/path/to/sources/"
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curl "http://localhost:8888/path/to/sources/README.md"
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# POST a file with a new name and read it back
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curl -F "filename=@Makefile" "http://localhost:8888/path/to/sources/new_name"
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curl "http://localhost:8888/path/to/sources/new_name"
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Limitation
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############
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List sub folders and files are not supported because Redis or Cassandra
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does not support prefix search.
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Flat Namespace Design
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############
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In stead of using both directory and file metadata, this implementation uses
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a flat namespace.
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If storing each directory metadata separatedly, there would be multiple
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network round trips to fetch directory information for deep directories,
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impeding system performance.
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A flat namespace would take more space because the parent directories are
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repeatedly stored. But disk space is a lesser concern especially for
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distributed systems.
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So either Redis or Cassandra is a simple file_full_path ~ file_id mapping.
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(Actually Cassandra is a file_full_path ~ list_of_file_ids mapping
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with the hope to support easy file appending for streaming files.)
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Complexity
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###################
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For one file retrieval, the full_filename=>file_id lookup will be O(logN)
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using Redis or Cassandra. But very likely the one additional network hop would
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take longer than the actual lookup.
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Use Cases
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#########################
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Clients can assess one "weed filer" via HTTP, create files via HTTP POST,
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read files via HTTP POST directly.
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Future
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###################
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SeaweedFS can support other distributed databases. It will be better
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if that database can support prefix search, in order to list files
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under a directory.
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Helps Wanted
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########################
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Please implement your preferred metadata store!
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Just follow the cassandra_store/cassandra_store.go file and send me a pull
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request. I will handle the rest.
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