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16. Cache Digests

Cache Digest FAQs compiled by Niall Doherty.

16.1 What is a Cache Digest?

A Cache Digest is a summary of the contents of an Internet Object Caching Server. It contains, in a compact (i.e. compressed) format, an indication of whether or not particular URLs are in the cache.

A "lossy" technique is used for compression, which means that very high compression factors can be achieved at the expense of not having 100% correct information.

16.2 How and why are they used?

Cache servers periodically exchange their digests with each other.

When a request for an object (URL) is received from a client a cache can use digests from its peers to find out which of its peers (if any) have that object. The cache can then request the object from the closest peer (Squid uses the NetDB database to determine this).

Note that Squid will only make digest queries in those digests that are enabled. It will disable a peers digest IFF it cannot fetch a valid digest for that peer. It will enable that peers digest again when a valid one is fetched.

The checks in the digest are very fast and they eliminate the need for per-request queries to peers. Hence:

  • Latency is eliminated and client response time should be improved.
  • Network utilisation may be improved.

Note that the use of Cache Digests (for querying the cache contents of peers) and the generation of a Cache Digest (for retrieval by peers) are independent. So, it is possible for a cache to make a digest available for peers, and not use the functionality itself and vice versa.

16.3 What is the theory behind Cache Digests?

Cache Digests are based on Bloom Filters - they are a method for representing a set of keys with lookup capabilities; where lookup means "is the key in the filter or not?".

In building a cache digest:

  • A vector (1-dimensional array) of m bits is allocated, with all bits initially set to 0.
  • A number, k, of independent hash functions are chosen, h1, h2, ..., hk, with range { 1, ..., m } (i.e. a key hashed with any of these functions gives a value between 1 and m inclusive).
  • The set of n keys to be operated on are denoted by: A = { a1, a2, a3, ..., an }.

Adding a Key

To add a key the value of each hash function for that key is calculated. So, if the key was denoted by a, then h1(a), h2(a), ..., hk(a) are calculated.

The value of each hash function for that key represents an index into the array and the corresponding bits are set to 1. So, a digest with 6 hash functions would have 6 bits to be set to 1 for each key added.

Note that the addition of a number of different keys could cause one particular bit to be set to 1 multiple times.

Querying a Key

To query for the existence of a key the indices into the array are calculated from the hash functions as above.

  • If any of the corresponding bits in the array are 0 then the key is not present.
  • If all of the corresponding bits in the array are 1 then the key is likely to be present.

Note the term likely. It is possible that a collision in the digest can occur, whereby the digest incorrectly indicates a key is present. This is the price paid for the compact representation. While the probability of a collision can never be reduced to zero it can be controlled. Larger values for the ratio of the digest size to the number of entries added lower the probability. The number of hash functions chosen also influence the probability.

Deleting a Key

To delete a key, it is not possible to simply set the associated bits to 0 since any one of those bits could have been set to 1 by the addition of a different key!

Therefore, to support deletions a counter is required for each bit position in the array. The procedures to follow would be:

  • When adding a key, set appropriate bits to 1 and increment the corresponding counters.
  • When deleting a key, decrement the appropriate counters (while > 0), and if a counter reaches 0 then the corresponding bit is set to 0.

16.4 How is the size of the Cache Digest in Squid determined?

Upon initialisation, the capacity is set to the number of objects that can be (are) stored in the cache. Note that there are upper and lower limits here.

An arbitrary constant, bits_per_entry (currently set to 5), is used to calculate the size of the array using the following formula:

  number of bits in array = capacity * bits_per_entry + 7

The size of the digest, in bytes, is therefore:

  digest size = int (number of bits in array / 8)

When a digest rebuild occurs, the change in the cache size (capacity) is measured. If the capacity has changed by a large enough amount (10%) then the digest array is freed and reallocated memory, otherwise the same digest is re-used.

16.5 What hash functions (and how many of them) does Squid use?

The protocol design allows for a variable number of hash functions (k). However, Squid employs a very efficient method using a fixed number - four.

Rather than computing a number of independent hash functions over a URL Squid uses a 128-bit MD5 hash of the key (actually a combination of the URL and the HTTP retrieval method) and then splits this into four equal chunks.

Each chunk, modulo the digest size (m), is used as the value for one of the hash functions - i.e. an index into the bit array.

Note: As Squid retrieves objects and stores them in its cache on disk, it adds them to the in-RAM index using a lookup key which is an MD5 hash - the very one discussed above. This means that the values for the Cache Digest hash functions are already available and consequently the operations are extremely efficient!

Obviously, modifying the code to support a variable number of hash functions would prove a little more difficult and would most likely reduce efficiency.

16.6 How are objects added to the Cache Digest in Squid?

Every object referenced in the index in RAM is checked to see if it is suitable for addition to the digest.

A number of objects are not suitable, e.g. those that are private, not cachable, negatively cached etc. and are skipped immediately.

A freshness test is next made in an attempt to guess if the object will expire soon, since if it does, it is not worthwhile adding it to the digest. The object is checked against the refresh patterns for staleness...

Since Squid stores references to objects in its index using the MD5 key discussed earlier there is no URL actually available for each object - which means that the pattern used will fall back to the default pattern, ".". This is an unfortunate state of affairs, but little can be done about it. A cd_refresh_pattern option will be added to the configuration file soon which will at least make the confusion a little clearer :-)

Note that it is best to be conservative with your refresh pattern for the Cache Digest, i.e. do not add objects if they might become stale soon. This will reduce the number of False Hits.

16.7 Does Squid support deletions in Cache Digests? What are diffs/deltas?

Squid does not support deletions from the digest. Because of this the digest must, periodically, be rebuilt from scratch to erase stale bits and prevent digest pollution.

A more sophisticated option is to use diffs or deltas. These would be created by building a new digest and comparing with the current/old one. They would essentially consist of aggregated deletions and additions since the previous digest.

Since less bandwidth should be required using these it would be possible to have more frequent updates (and hence, more accurate information).


  • RAM - extra RAM needed to hold two digests while comparisons takes place.
  • CPU - probably a negligible amount.

16.8 When and how often is the local digest built?

The local digest is built:

  • when store_rebuild completes after startup (the cache contents have been indexed in RAM), and
  • periodically thereafter. Currently, it is rebuilt every hour (more data and experience is required before other periods, whether fixed or dynamically varying, can "intelligently" be chosen). The good thing is that the local cache decides on the expiry time and peers must obey (see later).

While the [new] digest is being built in RAM the old version (stored on disk) is still valid, and will be returned to any peer requesting it. When the digest has completed building it is then swapped out to disk, overwriting the old version.

The rebuild is CPU intensive, but not overly so. Since Squid is programmed using an event-handling model, the approach taken is to split the digest building task into chunks (i.e. chunks of entries to add) and to register each chunk as an event. If CPU load is overly high, it is possible to extend the build period - as long as it is finished before the next rebuild is due!

It may prove more efficient to implement the digest building as a separate process/thread in the future...

16.9 How are Cache Digests transferred between peers?

Cache Digests are fetched from peers using the standard HTTP protocol (note that a pull rather than push technique is used).

After the first access to a peer, a peerDigestValidate event is queued (this event decides if it is time to fetch a new version of a digest from a peer). The queuing delay depends on the number of peers already queued for validation - so that all digests from different peers are not fetched simultaneously.

A peer answering a request for its digest will specify an expiry time for that digest by using the HTTP Expires header. The requesting cache thus knows when it should request a fresh copy of that peers digest.

Note: requesting caches use an If-Modified-Since request in case the peer has not rebuilt its digest for some reason since the last time it was fetched.

16.10 How and where are Cache Digests stored?

Cache Digest built locally

Since the local digest is generated purely for the benefit of its neighbours keeping it in RAM is not strictly required. However, it was decided to keep the local digest in RAM partly because of the following:

  • Approximately the same amount of memory will be (re-)allocated on every rebuild of the digest,
  • the memory requirements are probably quite small (when compared to other requirements of the cache server),
  • if ongoing updates of the digest are to be supported (e.g. additions/deletions) it will be necessary to perform these operations on a digest in RAM, and
  • if diffs/deltas are to be supported the "old" digest would have to be swapped into RAM anyway for the comparisons.

When the digest is built in RAM, it is then swapped out to disk, where it is stored as a "normal" cache item - which is how peers request it.

Cache Digest fetched from peer

When a query from a client arrives, fast lookups are required to decide if a request should be made to a neighbour cache. It it therefore required to keep all peer digests in RAM.

Peer digests are also stored on disk for the following reasons:

  • Recovery - If stopped and restarted, peer digests can be reused from the local on-disk copy (they will soon be validated using an HTTP IMS request to the appropriate peers as discussed earlier), and
  • Sharing - peer digests are stored as normal objects in the cache. This allows them to be given to neighbour caches.

16.11 How are the Cache Digest statistics in the Cache Manager to be interpreted?

Cache Digest statistics can be seen from the Cache Manager or through the client utility. The following examples show how to use the client utility to request the list of possible operations from the localhost, local digest statistics from the localhost, refresh statistics from the localhost and local digest statistics from another cache, respectively.

  ./client mgr:menu

  ./client mgr:store_digest

  ./client mgr:refresh

  ./client -h peer mgr:store_digest

The available statistics provide a lot of useful debugging information. The refresh statistics include a section for Cache Digests which explains why items were added (or not) to the digest.

The following example shows local digest statistics for a 16GB cache in a corporate intranet environment (may be a useful reference for the discussion below).

store digest: size: 768000 bytes
         entries: count: 588327 capacity: 1228800 util: 48%
         deletion attempts: 0
         bits: per entry: 5 on: 1953311 capacity: 6144000 util: 32%
         bit-seq: count: 2664350 avg.len: 2.31
         added: 588327 rejected: 528703 ( 47.33 %) del-ed: 0
         collisions: on add: 0.23 % on rej: 0.23 %

entries:capacity is a measure of how many items "are likely" to be added to the digest. It represents the number of items that were in the local cache at the start of digest creation - however, upper and lower limits currently apply. This value is multiplied by bits: per entry (an arbitrary constant) to give bits:capacity, which is the size of the cache digest in bits. Dividing this by 8 will give store digest: size which is the size in bytes.

The number of items represented in the digest is given by entries:count. This should be equal to added minus deletion attempts.

Since (currently) no modifications are made to the digest after the initial build (no additions are made and deletions are not supported) deletion attempts will always be 0 and entries:count should simply be equal to added.

entries:util is not really a significant statistic. At most it gives a measure of how many of the items in the store were deemed suitable for entry into the cache compared to how many were "prepared" for.

rej shows how many objects were rejected. Objects will not be added for a number of reasons, the most common being refresh pattern settings. Remember that (currently) the default refresh pattern will be used for checking for entry here and also note that changing this pattern can significantly affect the number of items added to the digest! Too relaxed and False Hits increase, too strict and False Misses increase. Remember also that at time of validation (on the peer) the "real" refresh pattern will be used - so it is wise to keep the default refresh pattern conservative.

bits: on indicates the number of bits in the digest that are set to 1. bits: util gives this figure as a percentage of the total number of bits in the digest. As we saw earlier, a figure of 50% represents the optimal trade-off. Values too high (say > 75%) would cause a larger number of collisions, and hence False Hits, while lower values mean the digest is under-utilised (using unnecessary RAM). Note that low values are normal for caches that are starting to fill up.

A bit sequence is an uninterrupted sequence of bits with the same value. bit-seq: avg.len gives some insight into the quality of the hash functions. Long values indicate problem, even if bits:util is 50% (> 3 = suspicious, > 10 = very suspicious).

16.12 What are False Hits and how should they be handled?

A False Hit occurs when a cache believes a peer has an object and asks the peer for it but the peer is not able to satisfy the request.

Expiring or stale objects on the peer are frequent causes of False Hits. At the time of the query actual refresh patterns are used on the peer and stale entries are marked for revalidation. However, revalidation is prohibited unless the peer is behaving as a parent, or miss_access is enabled. Thus, clients can receive error messages instead of revalidated objects!

The frequency of False Hits can be reduced but never eliminated completely, therefore there must be a robust way of handling them when they occur. The philosophy behind the design of Squid is to use lightweight techniques and optimise for the common case and robustly handle the unusual case (False Hits).

Squid will soon support the HTTP only-if-cached header. Requests for objects made to a peer will use this header and if the objects are not available, the peer can reply appropriately allowing Squid to recognise the situation. The following describes what Squid is aiming towards:

  • Cache Digests used to obtain good estimates of where a requested object is located in a Cache Hierarchy.
  • Persistent HTTP Connections between peers. There will be no TCP startup overhead and both latency and network load will be similar for ICP (i.e. fast).
  • HTTP False Hit Recognition using the only-if-cached HTTP header - allowing fall back to another peer or, if no other peers are available with the object, then going direct (or through a parent if behind a firewall).

16.13 How can Cache Digest related activity be traced/debugged?

Enabling Cache Digests

If you wish to use Cache Digests (available in Squid version 2) you need to add a configure option, so that the relevant code is compiled in:

  ./configure --enable-cache-digests ...

What do the access.log entries look like?

If a request is forwarded to a neighbour due a HIT in that neighbour's Cache Digest the hierarchy (9th) field of the access.log file for the local cache will look like CACHE_DIGEST_HIT/neighbour. The Log Tag (4th field) should obviously show a MISS.

On the peer cache the request should appear as a normal HTTP request from the first cache.

What does a False Hit look like?

The easiest situation to analyse is when two caches (say A and B) are involved neither of which uses the other as a parent. In this case, a False Hit would show up as a CACHE_DIGEST_HIT on A and NOT as a TCP_HIT on B (or vice versa). If B does not fetch the object for A then the hierarchy field will look like NONE/- (and A will have received an Access Denied or Forbidden message). This will happen if the object is not "available" on B and B does not have miss_access enabled for A (or is not acting as a parent for A).

How is the cause of a False Hit determined?

Assume A requests a URL from B and receives a False Hit

  • Using the client utility PURGE the URL from A, e.g.

      ./client -m PURGE 'URL'

  • Using the client utility request the object from A, e.g.

      ./client 'URL'

The HTTP headers of the request are available. Two header types are of particular interest:

  • X-Cache - this shows whether an object is available or not.
  • X-Cache-Lookup - this keeps the result of a store table lookup before refresh causing rules are checked (i.e. it indicates if the object is available before any validation would be attempted).

The X-Cache and X-Cache-Lookup headers from A should both show MISS.

If A requests the object from B (which it will if the digest lookup indicates B has it - assuming B is closest peer of course :-) then there will be another set of these headers from B.

If the X-Cache header from B shows a MISS a False Hit has occurred. This means that A thought B had an object but B tells A it does not have it available for retrieval. The reason why it is not available for retrieval is indicated by the X-Cache-Lookup header. If:

  • X-Cache-Lookup = MISS then either A's (version of B's) digest is out-of-date or corrupt OR a collision occurred in the digest (very small probability) OR B recently purged the object.
  • X-Cache-Lookup = HIT then B had the object, but refresh rules (or A's max-age requirements) prevent A from getting a HIT (validation failed).

Use The Source

If there is something else you need to check you can always look at the source code. The main Cache Digest functionality is organised as follows:

  • CacheDigest.c (debug section 70) Generic Cache Digest routines
  • store_digest.c (debug section 71) Local Cache Digest routines
  • peer_digest.c (debug section 72) Peer Cache Digest routines

Note that in the source the term Store Digest refers to the digest created locally. The Cache Digest code is fairly self-explanatory (once you understand how Cache Digests work):

16.14 What about ICP?


16.15 Is there a Cache Digest Specification?

There is now, thanks to Martin Hamilton and Alex Rousskov.

Cache Digests, as implemented in Squid 2.1.PATCH2, are described in CacheDigest/cache-digest-v5.txt.

You'll notice the format is similar to an Internet Draft. We decided not to submit this document as a draft because Cache Digests will likely undergo some important changes before we want to try to make it a standard.

16.16 Would it be possible to stagger the timings when cache_digests are retrieved from peers?

Note: The information here is current for version 2.2.

Squid already has code to spread the digest updates. The algorithm is currently controlled by a few hard-coded constants in peer_digest.c. For example, GlobDigestReqMinGap variable determines the minimum interval between two requests for a digest. You may want to try to increase the value of GlobDigestReqMinGap from 60 seconds to whatever you feel comfortable with (but it should be smaller than hour/number_of_peers, of course).

Note that whatever you do, you still need to give Squid enough time and bandwidth to fetch all the digests. Depending on your environment, that bandwidth may be more or less than an ICP would require. Upcoming digest deltas (x10 smaller than the digests themselves) may be the only way to solve the ``big scale'' problem.

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