seaweedfs/docs/distributed_filer.rst

Distributed Filer
===========================

The default weed filer is in standalone mode, storing file metadata on disk.
It is quite efficient to go through deep directory path and can handle
millions of files.

However, no SPOF is a must-have requirement for many projects.

Luckily, SeaweedFS is so flexible that we can use a completely different way
to manage file metadata.

This distributed filer uses Cassandra to store the metadata.

Cassandra Setup
#####################
Here is the CQL to create the table.CassandraStore.
Optionally you can adjust the keyspace name and replication settings.
For production server, you would want to set replication_factor to 3.

.. code-block:: bash

	create keyspace seaweed WITH replication = {
	  'class':'SimpleStrategy',
	  'replication_factor':1
	};

	use seaweed;

	CREATE TABLE seaweed_files (
	   path varchar,
	   fids list<varchar>,
	   PRIMARY KEY (path)
	);


Sample usage
#####################

To start a weed filer in distributed mode:

.. code-block:: bash

	# assuming you already started weed master and weed volume
	weed filer -cassandra.server=localhost

Now you can add/delete files

.. code-block:: bash

	# POST a file and read it back
	curl -F "filename=@README.md" "http://localhost:8888/path/to/sources/"
	curl "http://localhost:8888/path/to/sources/README.md"
	# POST a file with a new name and read it back
	curl -F "filename=@Makefile" "http://localhost:8888/path/to/sources/new_name"
	curl "http://localhost:8888/path/to/sources/new_name"

Limitation
############
List sub folders and files are not supported because Cassandra does not support
prefix search.

Flat Namespace Design
############
In stead of using both directory and file metadata, this implementation uses
a flat namespace.

If storing each directory metadata separatedly, there would be multiple
network round trips to fetch directory information for deep directories,
impeding system performance.

A flat namespace would take more space because the parent directories are
repeatedly stored. But disk space is a lesser concern especially for
distributed systems.

The Cassandra table is a simple file_full_path ~ file_id mapping. Actually
it is a file_full_path ~ list_of_file_ids mapping with the hope to support
easy file appending for streaming files.

Complexity
###################

For one file retrieval, the full_filename=>file_id lookup will be O(logN)
using Cassandra. But very likely the one additional network hop would
take longer than the Cassandra internal lookup.

Use Cases
#########################

Clients can assess one "weed filer" via HTTP, create files via HTTP POST,
read files via HTTP POST directly.

Future
###################

SeaweedFS can support additional distributed databases. It will be better
if that database can support prefix search, in order to list files
under a directory.

Helps Wanted
########################

Please implement your preferred metadata store!

Just follow the cassandra_store/cassandra_store.go file and send me a pull
request. I will handle the rest.