Assume that you and I both read the same
row from the Customer table, we both change the data, and then we both
try to write our new versions back to the database.
Whose changes should be saved? Yours?
Mine? Neither?
A combination?
Similarly, if we both work with the same Customer
object stored in a shared object cache and try to make changes to it, what
should happen?
This article presents a practical overview of
concurrency control (for
theoretical discussions,
click here). Concurrency control deals with the issues involved with
allowing multiple people simultaneous access to shared entities, be they
objects, data records, or some other representation.
To
understand how to implement concurrency control within your system you must
start by understanding the basics of collisions -
you can either avoid them or detect and then resolve them.
The next step is to understand
transactions,
which are collections of actions that potentially modify two or more entities. An important message of this article is
that on modern software development projects that concurrency control and
transactions are not simply the domain of databases, instead they are issues
that are potentially pertinent to all of your architectural tiers.
In this article, I discuss:
- Introduction to collisions
- Locking strategies
- Collision resolution
strategies
- Choosing a locking strategy
In Implementing
Referential Integrity and Shared Business Logic I discuss the referential
integrity challenges that result from there being an object schema that is
mapped to a data schema, something that I like to call cross-schema referential
integrity problems. With respect to
collisions things are a little simpler, we only need to worry about the issues
with ensuring the consistency of entities within the system of record.
The system of record is the location where the official
version of an entity is located. This
is often data stored within a relational database although other
representations, such as an XML structure or an object, are also viable.
A collision is said to occur when two activities, which may or may
not be full-fledged
transactions, attempt to change entities within a system of
record. There are three fundamental
ways (Celko
1999) that two activities can interfere with one another:
-
Dirty read. Activity 1 (A1) reads an entity
from the system of record and then updates the system of record but does not
commit the change (for example, the change hasn't been finalized). Activity
2 (A2) reads the entity, unknowingly making a copy of the uncommitted
version. A1 rolls back (aborts) the changes, restoring the entity to the
original state that A1 found it in. A2 now has a version of the entity that
was never committed and therefore is not considered to have actually
existed.
-
Non-repeatable read. A1 reads an entity from
the system of record, making a copy of it. A2 deletes the entity from the
system of record. A1 now has a copy of an entity that does not officially
exist.
-
Phantom read. A1 retrieves a collection of
entities from the system of record, making copies of them, based on some
sort of search criteria such as "all customers with first name Bill."A2 then
creates new entities, which would have met the search criteria (for example,
inserts "Bill Klassen" into the database), saving them to the system of
record. If A1 reapplies the search criteria it gets a different result set.
Collisions will occur the more that data is allowed to go
stale in a cache and the more concurrent users/threads you have.
So what can you do? First,
you can take a pessimistic locking approach
that avoids collisions but reduces system performance.
Second, you can use an optimistic locking
strategy that enables you to detect collisions so you can resolve
them. Third, you can take an overly
optimistic locking strategy that ignores the issue completely.
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Pessimistic locking is an
approach where an
entity is locked in the database for the entire time that it is in application
memory (often in the form of an object). A lock
either limits or prevents other users from working with the entity in the
database. A write lock indicates
that the holder of the lock intends to update the entity and disallows anyone
from reading, updating, or deleting the entity.
A read lock indicates that the holder of the lock does not want the
entity to change while the hold the lock, allowing others to read the entity but
not update or delete it. The scope
of a lock might be the entire database, a table, a collection of rows, or a
single row. These types of locks
are called database locks, table locks, page locks, and row locks respectively.
The advantages of pessimistic locking are that it is easy to
implement and guarantees that your changes to the database are made consistently
and safely. The primary
disadvantage is that this approach isn't scalable.
When a system has many users, or when the transactions
involve a greater number of entities, or when transactions are long lived, then
the chance of having to wait for a lock to be released increases.
Therefore this limits the practical number of simultaneous users that
your system can support.
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With multi-user systems it is quite common to be in a situation
where collisions are infrequent. Although
the two of us are working with Customer objects, you're working with
the Wayne Miller object while I work with the John Berg object and therefore we
won't collide. When this is the
case optimistic locking becomes a viable concurrency control strategy.
The idea is that you accept the fact that collisions occur infrequently,
and instead of trying to prevent them you simply choose to detect them and then
resolve the collision when it does occur.
Figure 1 depicts the logic
for updating an object when optimistic locking is used.
The application reads the object into memory.
To do this a read lock is obtained on the data, the data is read into
memory, and the lock is released. At
this point in time the row(s) may be marked to facilitate detection of a
collision (more on this later). The
application then manipulates the object until the point that it needs to be
updated. The application then
obtains a write lock on the data and reads the original source back so as to
determine if there's been a collision. The
application determines that there has not been a collision so it updates the
data and unlocks it. Had a
collision been detected, e.g. the data had been updated by another process after
it had originally been read into memory, then the collision would need to be
resolved.
Figure 1. Updating an object following
an optimistic locking approach.
There are two basic strategies for determining if a
collision has occurred:
-
Mark the source with a unique identifier.
The source data row is marked with a unique value each time it is updated.
At the point of update, the mark is checked, and if there is a different
value than what you originally read in, then you know that there has been an
update to the source. There are different types of concurrency marks:
-
Datetime stamps (the database server should assign
this value because you can't count on the time clocks of all machines to
be in sync).
-
Incremental counters.
-
User IDs (this only works if everyone has a unique
ID and you're logged into only one machine and the applications ensure
that only one copy of an object exists in memory).
-
Values generated by a globally unique
surrogate key
generator.
-
Retain a copy of the original.
The source data is retrieved at the point of updating and compared with the
values that were originally retrieved. If the values have changed, then a
collision has occurred. This strategy may be your only option if you are
unable to add sufficient columns to your database schema to maintain the
concurrency marks.
Figure 1 depicts a
naïve approach, and in fact there are ways to reduce the number of database
interactions.
The first three requests to the database - the initial
lock, marking (if appropriate) the source data, and unlocking - can be
performed as a single transaction. The
next two interactions, to lock and obtain a copy of the source data, can easily
be combined as a single trip to the database.
Furthermore the update and unlock can similarly be combined.
Another way to improve this is to combine the last four interactions into
a single transaction and simply perform collision detection on the database
server instead of the application server.
With the strategy you neither try to avoid nor detect collisions, assuming
that they will never occur. This
strategy is appropriate for single user systems, systems where the system of
record is guaranteed to be accessed by only one user or system process at a
time, or read-only tables. These
situations do
occur. It is important to recognize
that this strategy is completely inappropriate for multi-user systems.
You have five basic strategies that you can apply to
resolve collisions:
-
Give up.
-
Display the problem and let the user decide.
-
Merge the changes.
-
Log the problem so someone can decide later.
-
Ignore the collision and overwrite.
It is important to recognize that the granularity of a
collision counts. Assume that both of us are working with a copy of the
same Customer entity. If you
update a customer's name and I update their shopping preferences, then we can
still recover from this collision. In
effect the collision occurred at the entity level, we updated the same customer,
but not at the attribute level. It
is very common to detect potential collisions at the entity level then get smart
about resolving them at the attribute level.
For simplicity's sake, many project teams will choose a single locking
strategy and apply it for all tables. This works well when all, or at
least most, tables in your application have the same access characteristics.
However, for more complex applications you will likely need to implement several
locking strategies based on the access characteristics of individual tables.
One approach, suggested by Willem Bogaerts, is to categorize each table by type
to provide guidance as to a locking strategy for it. Strategies for doing
so are described in Table 1.
Table 1. Locking strategies by table type.
| Table Type |
Examples |
Suggested Locking Strategy |
| Live-High Volume |
|
|
| Live-Low Volume |
- Customer
- Insurance Policy
|
|
| Log (typically append only) |
- AccessLog
- AccountHistory
- TransactionRecord
|
|
| Lookup/Reference (typically read only) |
|
|
5. Acknowledgements
I'd like to thank Willem Bogaerts and Rich Stone for their feedback regarding
this article.
