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# General Information

## Terminology

S3QL can store data at different service providers and using different protocols. The term backend refers to both the part of S3QL that implements communication with a specific storage service and the storage service itself. Most backends can hold more than one S3QL file system and thus require some additional information that specifies the file system location within the backend. This location is called a bucket (for historical reasons).

Many S3QL commands expect a storage url as a parameter. A storage url specifies both the backend and the bucket and thus uniquely identifies an S3QL file system. The form of the storage url depends on the backend and is described together with the Storage Backends.

## Storing Authentication Information

Normally, S3QL reads username and password for the backend as well as an encryption passphrase for the bucket from the terminal. Most commands also accept an `--authfile` parameter that can be used to read this information from a file instead.

The authentication file consists of sections, led by a `[section]` header and followed by `name: value` entries. The section headers themselves are not used by S3QL but have to be unique within the file.

In each section, the following entries can be defined:

storage-url: | Specifies the storage url to which this section applies. If a storage url starts with the value of this entry, the section is considered applicable.
backend-login: | Specifies the username to use for authentication with the backend.
backend-password:
| Specifies the password to use for authentication with the backend.
bucket-passphrase:
| Specifies the passphrase to use to decrypt the bucket (if it is encrypted).

When reading the authentication file, S3QL considers every applicable section in order and uses the last value that it found for each entry. For example, consider the following authentication file:

    [s3]
    storage-url: s3://
    backend-login: joe
    backend-password: notquitesecret

    [bucket1]
    storage-url: s3://joes-first-bucket
    bucket-passphrase: neitheristhis

    [bucket2]
    storage-url: s3://joes-second-bucket
    bucket-passphrase: swordfish

    [bucket3]
    storage-url: s3://joes-second-bucket/with-prefix
    backend-login: bill
    backend-password: bi23ll
    bucket-passphrase: ll23bi

With this authentication file, S3QL would try to log in as “joe” whenever the s3 backend is used, except when accessing a storage url that begins with “s3://joes-second-bucket/with-prefix”. In that case, the last section becomes active and S3QL would use the “bill” credentials. Furthermore, bucket encryption passphrases will be used for storage urls that start with “s3://joes-first-bucket” or “s3://joes-second-bucket”.

The authentication file is parsed by the Python ConfigParser module.

## On Backend Reliability

S3QL has been designed for use with a storage backend where data loss is so infrequent that it can be completely neglected (e.g. the Amazon S3 backend). If you decide to use a less reliable backend, you should keep the following warning in mind and read this section carefully.

Warning

S3QL is not able to compensate for any failures of the backend. In particular, it is not able reconstruct any data that has been lost or corrupted by the backend. The persistence and durability of data stored in an S3QL file system is limited and determined by the backend alone.

On the plus side, if a backend looses or corrupts some of the stored data, S3QL will detect the problem. Missing data will be detected when running `fsck.s3ql` or when attempting to access the data in the mounted file system. In the later case you will get an IO Error, and the S3QL mount point will become inaccessible.

`fsck.s3ql` will report all the affected files and move them into the `/lost+found` directory of the file system.

You should be aware that, because of S3QL’s data de-duplication feature, the consequences of a data loss in the backend can be significantly more severe than you may expect. More concretely, a data loss in the backend at time x may cause data that is written after time x to be lost as well. What may happen is this:

1. You store an important file in the S3QL file system.
2. The backend looses the data blocks of this file. As long as you do not access the file or run `fsck.s3ql`, S3QL is not aware that the data has been lost by the backend.
3. You save an additional copy of the important file in a different location on the same S3QL file system.
4. S3QL detects that the contents of the new file are identical to the data blocks that have been stored earlier. Since at this point S3QL is not aware that these blocks have been lost by the backend, it does not save another copy of the file contents in the backend but relies on the (presumably) existing blocks instead.
5. Therefore, even though you saved another copy, you still do not have a backup of the important file (since both copies refer to the same data blocks that have been lost by the backend).

As one can see, this effect becomes the less important the more often one runs `fsck.s3ql`, since `fsck.s3ql` will make S3QL aware of any blocks that the backend may have lost. Figuratively, this establishes a “checkpoint”: data loss in the backend that occurred before running `fsck.s3ql` can not affect any file system operations performed after running `fsck.s3ql`.

Nevertheless, the recommended way to use S3QL is in combination with a sufficiently reliable storage backend. In that case none of the above will ever be a concern.

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