tahoe-lafs/docs/proposed/leasedb.rst

Lease database design

The target audience for this document is developers who wish to understand the new lease database (leasedb) planned to be added in Tahoe-LAFS v1.11.0.

Introduction

A "lease" is a request by an account that a share not be deleted before a specified time. Each storage server stores leases in order to know which shares to spare from garbage collection.

Motivation

The leasedb will replace the current design in which leases are stored in the storage server's share container files. That design has several disadvantages:

• Updating a lease requires modifying a share container file (even for immutable shares). This complicates the implementation of share classes. The mixing of share contents and lease data in share files also led to a security bug (ticket #1528).
• When only the disk backend is supported, it is possible to read and update leases synchronously because the share files are stored locally to the storage server. For the cloud backend, accessing share files requires an HTTP request, and so must be asynchronous. Accepting this asynchrony for lease queries would be both inefficient and complex. Moving lease information out of shares and into a local database allows lease queries to stay synchronous.

Also, the current cryptographic protocol for renewing and cancelling leases (based on shared secrets derived from secure hash functions) is complex, and the cancellation part was never used.

The leasedb solves the first two problems by storing the lease information in a local database instead of in the share container files. The share data itself is still held in the share container file.

At the same time as implementing leasedb, we devised a simpler protocol for allocating and cancelling leases: a client can use a public key digital signature to authenticate access to a foolscap object representing the authority of an account. This protocol is not yet implemented; at the time of writing, only an "anonymous" account is supported.

The leasedb also provides an efficient way to get summarized information, such as total space usage of shares leased by an account, for accounting purposes.

Design constraints

A share is stored as a collection of objects. The persistent storage may be remote from the server (for example, cloud storage).

Writing to the persistent store objects is in general not an atomic operation. So the leasedb also keeps track of which shares are in an inconsistent state because they have been partly written. (This may change in future when we implement a protocol to improve atomicity of updates to mutable shares.)

Leases are no longer stored in shares. The same share format is used as before, but the lease slots are ignored, and are cleared when rewriting a mutable share. The new design also does not use lease renewal or cancel secrets. (They are accepted as parameters in the storage protocol interfaces for backward compatibility, but are ignored. Cancel secrets were already ignored due to the fix for #1528.)

The new design needs to be fail-safe in the sense that if the lease database is lost or corruption is detected, no share data will be lost (even though the metadata about leases held by particular accounts has been lost).

Accounting crawler

A "crawler" is a long-running process that visits share container files at a slow rate, so as not to overload the server by trying to visit all share container files one after another immediately.

The accounting crawler replaces the previous "lease crawler". It examines each share container file and compares it with the state of the leasedb, and may update the state of the share and/or the leasedb.

The accounting crawler may perform the following functions (but see ticket #1834 for a proposal to reduce the scope of its responsibility):

• Remove leases that are past their expiration time. (Currently, this is done automatically before deleting shares, but we plan to allow expiration to be performed separately for individual accounts in future.)
• Delete the objects containing unleased shares — that is, shares that have stable entries in the leasedb but no current leases (see below for the definition of "stable" entries).
• Discover shares that have been manually added to storage, via scp or some other out-of-band means.
• Discover shares that are present when a storage server is upgraded to a leasedb-supporting version from a previous version, and give them "starter leases".
• Recover from a situation where the leasedb is lost or detectably corrupted. This is handled in the same way as upgrading from a previous version.
• Detect shares that have unexpectedly disappeared from storage. The disappearance of a share is logged, and its entry and leases are removed from the leasedb.

Accounts

An account holds leases for some subset of shares stored by a server. The leasedb schema can handle many distinct accounts, but for the time being we create only two accounts: an anonymous account and a starter account. The starter account is used for leases on shares discovered by the accounting crawler; the anonymous account is used for all other leases.

The leasedb has at most one lease entry per account per (storage_index, shnum) pair. This entry stores the times when the lease was last renewed and when it is set to expire (if the expiration policy does not force it to expire earlier), represented as Unix UTC-seconds-since-epoch timestamps.

For more on expiration policy, see ../garbage-collection.

Share states

The leasedb holds an explicit indicator of the state of each share.

The diagram and descriptions below give the possible values of the "state" indicator, what that value means, and transitions between states, for any (storage_index, shnum) pair on each server:

#        STATE_STABLE -------.
#         ^   |   ^ |         |
#         |   v   | |         v
#    STATE_COMING | |    STATE_GOING
#         ^       | |         |
#         |       | v         |
#         '----- NONE <------'

NONE: There is no entry in the shares table for this (storage_index, shnum) in this server's leasedb. This is the initial state.

STATE_COMING: The share is being created or (if a mutable share) updated. The store objects may have been at least partially written, but the storage server doesn't have confirmation that they have all been completely written.

STATE_STABLE: The store objects have been completely written and are not in the process of being modified or deleted by the storage server. (It could have been modified or deleted behind the back of the storage server, but if it has, the server has not noticed that yet.) The share may or may not be leased.

STATE_GOING: The share is being deleted.

State transitions

• STATE_GOING → NONE

  trigger: The storage server gains confidence that all store objects for the share have been removed.

  implementation:

  1. Remove the entry in the leasedb.

• STATE_STABLE → NONE

  trigger: The accounting crawler noticed that all the store objects for this share are gone.

  implementation:

  1. Remove the entry in the leasedb.

• NONE → STATE_COMING

  triggers: A new share is being created, as explicitly signalled by a client invoking a creation command, or the accounting crawler discovers an incomplete share.

  implementation:

  1. Add an entry to the leasedb with STATE_COMING.
  2. (In case of explicit creation) begin writing the store objects to hold the share.

• STATE_STABLE → STATE_COMING

  trigger: A mutable share is being modified, as explicitly signalled by a client invoking a modification command.

  implementation:

  1. Add an entry to the leasedb with STATE_COMING.
  2. Begin updating the store objects.

• STATE_COMING → STATE_STABLE

  trigger: All store objects have been written.

  implementation:

  1. Change the state value of this entry in the leasedb from STATE_COMING to STATE_STABLE.

• NONE → STATE_STABLE

  trigger: The accounting crawler discovers a complete share.

  implementation:

  1. Add an entry to the leasedb with STATE_STABLE.

• STATE_STABLE → STATE_GOING

  trigger: The share should be deleted because it is unleased.

  implementation:

  1. Change the state value of this entry in the leasedb from STATE_STABLE to STATE_GOING.
  2. Initiate removal of the store objects.

The following constraints are needed to avoid race conditions:

• While a share is being deleted (entry in STATE_GOING), we do not accept any requests to recreate it. That would result in add and delete requests for store objects being sent concurrently, with undefined results.
• While a share is being added or modified (entry in STATE_COMING), we treat it as leased.
• Creation or modification requests for a given mutable share are serialized.

Unresolved design issues

• What happens if a write to store objects for a new share fails permanently? If we delete the share entry, then the accounting crawler will eventually get to those store objects and see that their lengths are inconsistent with the length in the container header. This will cause the share to be treated as corrupted. Should we instead attempt to delete those objects immediately? If so, do we need a direct STATE_COMING → STATE_GOING transition to handle this case?
• What happens if only some store objects for a share disappear unexpectedly? This case is similar to only some objects having been written when we get an unrecoverable error during creation of a share, but perhaps we want to treat it differently in order to preserve information about the storage service having lost data.
• Does the leasedb need to track corrupted shares?

Future directions

Clients will have key pairs identifying accounts, and will be able to add leases for a specific account. Various space usage policies can be defined.

Better migration tools ('tahoe storage export'?) will create export files that include both the share data and the lease data, and then an import tool will both put the share in the right place and update the recipient node's leasedb.