Draft v0.1, May 21, 1997

1 Problem Description - Overview

Locking is used to arbitrate access to a resource amongst principals that have equal access rights to that resource.

This draft allows locks to vary over two parameters, the number of principals involved and the type of access to be granted. This draft will only provide for the definition of locking for one access type, write. However, the syntax is extensible enough to allow for the specification of other access types. It is a goal of this proposal that it use the same access verbs as will be defined in the access control draft.

1.1 Exclusive Vs. Shared Locks

The most basic form of LOCK is an exclusive lock. This is a lock where the access right in question is only granted to a single principal. The need for this arbitration results from a desire to avoid having to constantly merge results. In fact, users so dislike having to merge that they would rather serialize their access to a resource rather than have to constantly perform merges.

However, there are times when the goal of a lock is not to exclude others from exercising an access right but rather to provide a mechanism for principals to indicate that they intend to exercise their access right. Shared locks are provided for this case. A shared lock allows multiple principals to receive a lock, hence any principal with appropriate access can get the lock. However, if the principal does not get the lock they will not be able to alter the resource.

1.2 Required Support

A DAV compliant server is not required to support locking in any form. If the server does support locking it may choose to support any combination of exclusive and shared locks for any access types.

The reason for this flexibility is that server implementers have said that they are willing to accept minimum requirements on all services but locking. Locking policy strikes to the very heart of their resource management and versioning systems and they require control over what sort of locking will be made available. For example, some systems only support shared write locks while others only provide support for exclusive write locks. As each system is sufficiently different to merit exclusion of certain locking features, the authors are proposing that locking be allowed as the sole axis of negotiation within DAV.

2 LOCK Method

2.1 Operation

A lock method invocation creates the lock specified by the Lock-Info header on the request-URI. Lock method requests SHOULD NOT have a request body. A user-agent SHOULD submit an Owner header field with a lock request.

A successful response to a lock invocation MUST include a Lock-Token header. If the server supports a time based lock removal mechanism on the resource, a successful lock invocation SHOULD return a Time-Out header.

2.2 Write Lock

A write lock prevents a principal without the lock from successfully executing a PUT, POST, DELETE, MKCOL, PATCH, GETMETA, DELMETA, or ADDMETA on the locked resource. All other methods, GET in particular, function independent of the lock.

[Author's Note: If the ADDREF and DELREF methods survive (currently discussed in the container draft) then they should be included in the list of methods that can not be executed on a write locked resources by principals without the lock.]

It is possible to assert a write lock on a null resource in order to lock the name. Please note, however, that locking a null resource effectively makes the resource non-null as the resource now has lock related properties defined on it.

Write locking a container also prevents adding or removing members of the container. This means that attempts to PUT/POST a resource into the immediate name space of the write locked container MUST fail if the principal requesting the action does not have the write lock on the container. Furthermore if a principal different from the one with the write lock on the container successfully locks one of the container's members then the lock on the member MUST be restricted so as to prevent deletion. Thus the only way to delete a member of a write locked container is to be the owner of the write lock.

Note that the consequence of this behavior is that if a member is locked while the parent is unlocked, it will be impossible for anyone but the owner of the lock on the member to lock the parent. If this were not so then it would be possible for someone to modify the nature of a lock after it had been granted.

2.3 Lock Scope

A lock, by default, affects the entire state of the resource, including its associated properties. For containers, a lock also affects the ability to add or remove members. The nature of the effect depends upon the type of access control involved.

The default behavior when locking a container is to act as if a "Depth: 0" header (defined in the name space draft) had been placed on the method.

2.4 Locking Replicated Resources

Some servers automatically replicate resources across multiple URLs. In such a circumstance the server MAY only accept a lock on one of the URLs if the server can guarantee that the lock will be honored across all the URLs.

2.5 Interaction with other Methods

Only two methods, MOVE and DELETE, have side effects which involve locks. When a resource is moved, its lock SHOULD be moved with it. However this may not always be possible and there is currently no proposal to create a header which would specify that the lock request should fail if the resource's locks can not be maintained. Please note that a COPY does not copy any locks on the source resource over to the destination resource. Deleting a resource MUST remove all locks on the resource.

2.7 Lock Compatibility Table

The table below describes the behavior which occurs when a lock request is made on a resource.

Current lock state/ Lock request Shared Lock Exclusive Lock
Shared LockTrueFalse
Exclusive LockFalseFalse*

Legend: True = lock MAY be granted. False = lock MUST NOT be granted. *=if the principal requesting the lock is the owner of the lock, the lock MAY be regranted.

The current lock state of a resource is given in the leftmost column, and lock requests are listed in the first row. The intersection of a row and column gives the result of a lock request. For example, if a shared lock is held on a resource, and an exclusive lock is requested, the table entry is "false", indicating the lock must not be granted.

If an exclusive lock is re-requested by the principal who owns the lock, the lock MAY be regranted.

2.8 Status Codes

Still mostly TBD. Some potential error conditions are:

- Server will not accept lock tokens on any resource but the one it was defined on

- Client does not have sufficient access to use an if-* header with a lock token.

- Request failed because of a lock

- LOCK succeeded, but not a new lock, client already has this lock

2.9 Example

LOCK /workspace/webdav/proposal.doc HTTP/1.1
Host: webdav.sb.aol.com
Lock-Info: LockType=Write LockScope=Exclusive
Owner: <http://www.ics.uci.edu/~ejw/contact.html>

HTTP/1.1 200 OK
Lock-Token: StateToken:Type=^DAV:/LOCK/DAVLOCK^:
Time-Out: ClockType=Activity TimeType=second;604800

This example shows the successful creation of an exclusive write lock on resource http://webdav.sb.aol.com/workspace/webdav/proposal.doc. The resource http://www.ics.uci.edu/~ejw/contact.html contains contact information for the owner of the lock. The server has an activity-based timeout policy in place on this resource, which causes the lock to automatically be removed after 1 week (604800 seconds). The response has a Lock-Token header that gives the state token URL for the lock token generated by this lock request.

3 Lock Method Headers

3.1 Lock-Info Request Header

The Lock-Info header specifies the scope and type of a lock for a LOCK method request. The syntax specification below is extensible, allowing new type and scope identifiers to be added.

LockInfo = "Lock-Info" ":" LockType SP LockScope CRLF
LockType = "LockType" "=" ("Write" | *(uchar | reserved))
LockScope = "LockScope" "=" ("Exclusive" | *(uchar | reserved))

3.2 Owner Request Header

3.2.1 Problem Description

When discovering the list of owners of locks on a resource, a principal may want to be able to contact the owner directly. For this to be possible the lock discovery mechanism must provide enough information for the lock owner to be contacted.

3.2.2 Solution Requirements

Not all systems have authentication procedures that provide sufficient information to identify a particular user in a way that is meaningful to a human. In addition, many systems that do have sufficient information, such as a name and e-mail address, do not have the ability to hook this information into the lock discovery mechanism. Therefore a means is needed to allow principals to identify themselves in a manner which will be meaningful to a human.

The From header (defined in RFC 2068), which contains only an email mailbox, is not sufficient for the purposes of quick identification. When desperately looking for someone to remove a lock, e-mail is often not sufficient. A telephone number would be better. Furthermore, the email address in the From field may or may not support including the owners name and that name is often set to an alias anyway. As such a header more flexible than From is required.

3.2.3 Syntax

Owner = "Owner" ":" "<" URI ">" | quoted-string CRLF

The URI SHOULD provide a means for either directly contacting the principal, say a telephone or e-mail URI, or for finding out who the principal is, say the URL of a homepage. The quoted string SHOULD provide a means for directly contacting the principal, such as a name and telephone number.

3.3 Time-Out Response Header

3.3.1 Problem Description

In a perfect world principals take out locks, use the resource as needed, and then remove the lock when it is no longer needed. However, this scenario is frequently not completed, leaving active but unused locks. Reasons for this include client programs crashing and loosing information about locks, users leaving their systems for the day and forgetting to remove their locks, etc. As a result of this behavior, servers need to establish a policy by which they can remove a lock without input from the lock owner. Once such a policy is instituted, the server also needs a mechanism to inform the principal of the policy.

3.3.2 Solution Requirements

There are two basic lock removal policies, administrator and time based remove. In the first case a principal other than the lock owner has sufficient access rights to order the lock removed, even though they did not take it out. User-agents MUST assume that such a mechanism is available and thus locks may arbitrarily disappear at any time. If their actions require confirmation of the existence of a lock then the If-State headers are available.

The second solution, is the time based removal policy. These come in two flavors, absolute and activity.

Absolute systems work by setting a timer as soon as the lock is taken out. When the timer runs out, the lock is removed. Executing a second LOCK method can renew the lock.

Activity based systems set a timer as soon as the lock is taken out. Every time a method is executed on the resource, the timer is reset. If the timer runs out, the lock is removed.

[Author's Note: At the last author's meeting it was decided that both timer types should be supported. I now believe that is the wrong decision. The absolute lock is unlikely to be implemented as writing an effective client side interface to explain the lock would be awkward. As such I recommend it be removed.]

3.3.3 Syntax

TimeOut = "Time-Out" ":" ClockType SP TimeType CRLF
ClockType = "ClockType" "=" ("Activity" | "Absolute")
TimeType = "TimeType" "=" "Second" ";" (TimeOutVal)
TimeOutVal = 1*digit

The "Second" TimeType specifies the number of seconds which may elapse before the lock is automatically removed. A server MUST not generate a time out value for "second" greater than 2^32.

If no time based system is in use then a Time-Out header MUST NOT be returned. The Time-Out header MUST only be returned in a response to a LOCK request.

A client is not allowed to specify a time-out value when requesting a lock.

3.4 Lock-Token Header

Lock-Token = "Lock-Token" ":" Lock-Token-URL

The Lock-Token header is only returned in a successful response to the LOCK method or with the UNLOCK method.

4 Lock Tokens

4.1 Problem Description

It is possible that once a lock has been granted it may be removed without the lock owner's knowledge. This can cause serialization problems if the lock owner executes methods thinking their lock is still in effect. Thus a mechanism is needed for a principal to predicate the successful execution of a message upon the continuing existence of a lock.

4.2 Proposed Solution

The proposed solution is to provide a lock token in the response of a lock request. The lock token is a state token and describes a particular lock. The same lock token must never be repeated on a particular resource. This prevents problems with long held outstanding lock tokens being confused with newer tokens. This uniqueness requirement is the same as for e-tags. This requirement also allows for tokens to be submitted across resources and servers without fear of confusion.

4.3 Lock Token Definition

The lock token is returned in the Lock-Token header in responses to the LOCK method. The lock token can also be discovered through lock discovery on a resource.

Lock-Token-URL = "StateToken:" Type ":" Resources ":" State-Info
Type = "Type" "=" "^DAV:/LOCK/DAVLOCK^"
Resources = "Res" "=" 1*("^" Caret-Encoded-URI "^")
Caret-Encoded-URI = <This is a URI which has all "^"s % encoded.>
State-Info = LockScope ":" LockType ":" ServerID ; LockScope, LockType defined in Lock-Info header
ServerID = "ServerID" "=" *(uchar | reserved)

The ServerID is a field for use by the server. Its most basic purpose is to put in a unique identifier to guarantee that a server will never confuse an old lock token with a newer one. However the server is free to use the field to record whatever information it deems fit. The field is opaque to clients.

4.4 Wild Cards and Lock Tokens - Open Issue

It would seem useful to allow wildcards into the lock tokens so it is possible to submit requests such as "Only process this message if there are no write locks on the resource". That would look like:

If-None-State-Match: StateToken:Type=^DAV:/LOCK/DAVLOCK^:LockType=Write:*

However this feature is only useful if all servers are required to support it, otherwise clients will have to be written to first try and submit the request, have it fail, and then have to do lock discovery.

5 UNLOCK Method

5.1 Problem Definition

The UNLOCK method removes the lock identified by the Lock-Token header from Request-URI.

5.2 Example

UNLOCK /workspace/webdav/proposal.doc HTTP/1.1
Host: webdav.sb.aol.com
Lock-Token: StateToken:Type=^DAV:/LOCK/DAVLOCK^:

HTTP/1.1 200 OK

In this example, the lock from example of Section 2.9 is removed from the resource at http://webdav.sb.aol.com/workspace/webdav/proposal.doc

5.3 Status Codes


6 Discovery Mechanisms

6.1 Lock Type Discovery

6.1.1 Problem Definition

Since server lock support is optional, a client trying to lock a resource on a server can either try the lock and hope for the best or can perform some form of discovery to determine what lock types the server actually supports, then formulate a supported request. This is known as lock type discovery. Lock type discovery is not the same as discovering what access control types are supported, as there may be access control types without corresponding lock types.

6.1.2 DAV:/LOCK/LOCKTYPE Property

The DAV:/LOCK/LOCKTYPE property of a resource returns a listing of the combinations of scope and access types which may be specified in a lock request on the resource. The response will consist of a list of lock types and lock scopes roughly of the form 1#(LockType LockScope) or some equivalent. Note that the actual contents are themselves controlled by access controls so a server is not required to provide information the client is not authorized to see. The actual format will be specified once a response format type is settled upon.

6.2 Active Lock Discovery

6.2.1 Problem Definition

If another principal locks a resource that a principal wishes to access, it is useful for the first principal to be able to find out who the second principal is.

6.2.2 Solution Requirements

The lock discovery mechanism should provide a list of who has the resource locked, what locks they have, and what their lock tokens are. The lock tokens are useful in shared lock situations where two principals in particular may want to guarantee that they do not overwrite each other. The lock tokens are also useful for administrative purposes so that an administrator can remove a lock by referring to its token.

6.2.3 DAV:/LOCK/DISCOVERY Property

The DAV:/LOCK/DISCOVERY property returns a listing of who has a lock, what type of lock they have, the time out type and the time remaining on the time out, and the associated lock token. The server is free to withhold any or all of this information if the requesting principal does not have sufficient access rights to see the requested data.

[NOTE: The actual format will be specified once a response format type is settled upon.]