commit 65a983581959eb2e8b006d92679ac2e353f8bbc7 Author: Isis Lovecruft isis@torproject.org Date: Wed Oct 23 18:32:02 2013 +0000
Separate backend database improvements into their own proposal. --- .../XXX-bridgedb-database-improvements.txt | 260 ++++++++++++++++++++ 1 file changed, 260 insertions(+)
diff --git a/doc/proposals/XXX-bridgedb-database-improvements.txt b/doc/proposals/XXX-bridgedb-database-improvements.txt new file mode 100644 index 0000000..2d25bd2 --- /dev/null +++ b/doc/proposals/XXX-bridgedb-database-improvements.txt @@ -0,0 +1,260 @@ +# -*- coding: utf-8 ; mode: org -*- + +Filename: XXX-bridgedb-database-improvements.txt +Title: "Scalability and Stability Improvements to BridgeDB: Switching to a + Distributed Database System and RDBMS" +Author: Isis Agora Lovecruft +Created: 12 Oct 2013 +Related Proposals: XXX-social-bridge-distribution.txt +Status: Open + +* I. Overview + + BridgeDB is Tor's Bridge Distribution system, which currently has two major + Bridge Distribution mechanisms: the HTTPS Distributor and an Email + Distributor. [0] + + BridgeDB is written largely in Twisted Python, and uses Python2's builtin + sqlite3 as its database backend. Unfortunately, this backend system is + already showing strain through increased times for queries, and sqlite's + memory usage is not up-to-par with modern, more efficient, NoSQL databases. + + In order to better facilitate the implementation of newer, more complex + Bridge Distribution mechanisms, several improvements should be made to the + underlying database system of BridgeDB. Additionally, I propose that a + clear distinction in terms, as well as a modularisation of the codebase, be + drawn between the mechanisms for Bridge Distribution versus the backend + Bridge Database (BridgeDB) storage system. + + This proposal covers the design and implementation of a scalable NoSQL ― + Document-Based and Key-Value Relational ― database backend for storing data + on Tor Bridge relays, in an efficient manner that is ammenable to + interfacing with the Twisted Python asynchronous networking code of current + and future Bridge Distribution mechanisms. + +* II. Terminology + + BridgeDistributor := A program which decides when and how to hand out + information on a Tor Bridge relay, and to whom. + + BridgeDB := The backend system of databases and object-relational mapping + servers, which interfaces with the BridgeDistributor in order + to hand out bridges to clients, and to obtain and process new, + incoming ``@type bridge-server-descriptors``, + ``@type bridge-networkstatus`` documents, and + ``@type bridge-extrainfo`` descriptors. [3] + + BridgeFinder := A client-side program for an Onion Proxy (OP) which handles + interfacing with a BridgeDistributor in order to obtain new + Bridge relays for a client. A BridgeFinder also interfaces + with a local Tor Controller (such as TorButton or ARM) to + handle automatic, transparent Bridge configuration (no more + copy+pasting into a torrc) without being given any + additional privileges over the Tor process, [1] and relies + on the Tor Controller to interface with the user for + control input and displaying up-to-date information + regarding available Bridges, Pluggable Transport methods, + and potentially Invite Tickets and Credits (a cryptographic + currency without fiat value which is generated + automatically by clients whose Bridges remain largely + uncensored, and is used to purchase new Bridges), should a + Social Bridge Distributor be implemented. [2] + +* III. Databases +** III.A. Scalability Requirements + + Databases SHOULD be implemented in a manner which is ammenable to using a + distributed storage system; this is necessary because many potential + datatypes required by future BridgeDistributors MUST be stored permanently. + For example, in the designs for the Social Bridge Distributor, the list of + hash digests of spent Credits, and the list of hash digests of redeemed + Invite Tickets MUST be stored forever to prevent either from being replayed + ― or double-spent ― by a malicious user who wishes to block bridges faster. + Designing the BridgeDB backend system such that additional nodes may be + added in the future will allow the system to freely scale in relation to + the storage requirements of future BridgeDistributors. + + Additionally, requiring that the implementation allow for distributed + database backends promotes modularisation the components of BridgeDB, such + that BridgeDistributors can be separated from the backend storage system, + BridgeDB, as all queries will be issued through a simplified, common API, + regardless of the number of nodes system, or the design of future + BridgeDistributors. + +*** 1. Distributed Database System + + A distributed database system SHOULD be used for BridgeDB, in order to + scale resources as the number of Tor bridge users grows. This database + system, hereafter referred to as DDBS. + + The DDBS MUST be capable of working within Twisted's asynchronous + framework. If possible, a Object-Relational Mapper (ORM) SHOULD be used to + abstract the database backend's structure and query syntax from the Twisted + Python classes which interact with it, so that the type of database may be + swapped out for another with less code refactoring. + + The DDBM SHALL be used for persistent storage of complex data structures + such as the bridges, which MAY include additional information from both the + `@type bridge-server-descriptor`s and the `@type bridge-extra-info` + descriptors. [3] + +**** 1.a. Choice of DDBS + + CouchDB is chosen for its simple HTTP API, ease of use, speed, and official + support for Twisted Python applications. [4] Additionally, its + document-based data model is very similar to the current archetecture of + tor's Directory Server/Mirror system, in that an HTTP API is used to + retrieve data stored within virtual directories. Internally, it uses JSON + to store data and JavaScript as its query language, both of which are + likely friendlier to various other components of the Tor Metrics + infrastructure which sanitise and analyse portions of the Bridge + descriptors. At the very least, friendlier than hardcoding raw SQL queries + as Python strings. + +**** 1.b. Data Structures which should be stored in a DDBS: + + - RedactedDB - The Database of Blocked Bridges + + The RedactedDB will hold entries of bridges which have been discovered to + be unreachable from BridgeDB network vantage point, or have been reported + unreachable by clients. + + - BridgeDB - The Database of Bridges + + BridgeDB holds information on available Bridges, obtained via bridge + descriptors and networkstatus documents from the BridgeAuthority. Because + a Bridge may have multiple `ORPort`s and multiple + `ServerTransportListenAddress`es, attaching additional data to each of + these addresses which MAY include the following information on a blocking + event: + - Geolocational country code of the reported blocking event + - Timestamp for when the blocking event was first reported + - The method used for discovery of the block + - an the believed mechanism which is causing the block + would quickly become unwieldy, the RedactedDB and BridgeDB SHOULD be kept + separate. + + - User Credentials + + For the Social BridgeDistributor, these are rather complex, + increasingly-growing, concatenations (or tuples) of several datatypes, + including Non-Interactive Proofs-of-Knowledge (NIPK) of Commitments to + k-TAA Blind Signatures, and NIPK of Commitments to a User's current + number of Credits and timestamps of requests for Invite Tickets. + +*** 2. Key-Value Relational Database Mapping Server + + For simpler data structures which must be persistently stored, such as the + list of hashes of previously seen Invite Tickets, or the list of + previously spent Tokens, a Relational Database Mapping Server (RDBMS) + SHALL be used for optimisation of queries. + + Redis and Memcached are two examples of RDBMS which are well tested and + are known to work well with Twisted. The major difference between the two + is that Memcaches are stored only within volatile memory, while Redis + additionally supports commands for transferring objects into persistent, + on-disk storage. + + There are several support modules for interfacing with both Memcached and + Redis from Twisted Python, see Twisted's MemCacheProtocol class [5] [6] or + txyam [7] for Memcached, and txredis [8] or txredisapi [9] for + Redis. Additionally, numerous big name projects both use Redis as part of + their backend systems, and also provide helpful documentation on their own + experience of the process of switching over to the new systems. [17] For + non-Twisted Python Redis APIs, there is redis-py, which provides a + connection pool that could likely be interfaced with from Twisted Python + without too much difficultly. [10] [11] + +**** 2.a. Data Structures which should be stored in a RDBMS + + Simple, mostly-flat datatypes, and data which must be frequently indexed + should be stored in a RDBMS, such as large lists of hashes, or arbitrary + strings with assigned point-values (i.e. the "Uniform Mapping" for the + current HTTPS BridgeDistributor). + + For the Social BridgeDistributor, hash digests of the following datatypes + SHOULD be stored in the RDBMS, in order to prevent double-spending and + replay attacks: + + - Invite Tickets + + These are anonymous, unlinkable, unforgeable, and verifiable tokens + which are occasionally handed out to well-behaved Users by the Social + BridgeDistributor to permit new Users to be invited into the system. + When they are redeemed, the Social BridgeDistributor MUST store a hash + digest of their contents to prevent replayed Invite Tickets. + + - Spent Credits + + These are Credits which have already been redeemed for new Bridges. + The Social BridgeDistributor MUST also store a hash digest of Spent + Credits to prevent double-spending. + +*** 3. Bloom Filters and Other Database Optimisations + + In order to further decrease the need for lookups in the backend + databases, Bloom Filters can used to eliminate extraneous + queries. However, this optimization would only be beneficial for string + lookups, i.e. querying for a User's Credential, and SHOULD NOT be used for + queries within any of the hash lists, i.e. the list of hashes of + previously seen Invite Tickets. [14] + +**** 3.a. Bloom Filters within Redis + + It might be possible to use Redis' GETBIT and SETBIT commands to store a + Bloom Filter within a Redis cache system; [15] doing so would offload the + severe memory requirements of loading the Bloom Filter into memory in + Python when inserting new entries, reducing the time complexity from some + polynomial time complexity that is proportional to the integral of the + number of bridge users over the rate of change of bridge users over time, + to a time complexity of order O(1). + +**** 3.b. Expiration of Stale Data + + Some types of data SHOULD be safe to expire, such as User Credentials + which have not been updated within a certain timeframe. This idea should + be further explored to assess the safety and potential drawbacks to + removing old data. + + If there is data which SHOULD expire, the PEXPIREAT command provided by + Redis for the key datatype would allow the RDBMS itself to handle cleanup + of stale data automatically. [16] + +**** 4. Other potential uses of the improved Bridge database system + + Redis provides mechanisms for evaluations to be made on data by calling + the sha1 for a serverside Lua script. [15] While not required in the + slightest, it is a rather neat feature, as it would allow Tor's Metrics + infrastructure to offload some of the computational overhead of gathering + data on Bridge usage to BridgeDB (as well as diminish the security + implications of storing Bridge descriptors). + + Also, if Twisted's IProducer and IConsumer interfaces do not provide + needed interface functionality, or it is desired that other components of + the Tor software ecosystem be capable of scheduling jobs for BridgeDB, + there are well-tested mechanisms for using Redis as a message + queue/scheduling system. [16] + +* References + +[0]: https://bridges.torproject.org + mailto:bridges@bridges.torproject.org +[1]: See proposals 199-bridgefinder-integration.txt at + https://gitweb.torproject.org/torspec.git/blob/HEAD:/proposals/199-bridgefin... +[2]: See XXX-social-bridge-distribution.txt at + https://gitweb.torproject.org/user/isis/bridgedb.git/blob/refs/heads/feature... +[3]: https://metrics.torproject.org/formats.html#descriptortypes +[4]: https://github.com/couchbase/couchbase-python-client#twisted-api +[5]: https://twistedmatrix.com/documents/current/api/twisted.protocols.memcache.M... +[6]: http://stackoverflow.com/a/5162203 +[7]: http://findingscience.com/twisted/python/memcache/2012/06/09/txyam:-yet-anot... +[8]: https://pypi.python.org/pypi/txredis +[9]: https://github.com/fiorix/txredisapi +[10]: https://github.com/andymccurdy/redis-py/ +[11]: http://degizmo.com/2010/03/22/getting-started-redis-and-python/ +[12]: http://www.dr-josiah.com/2012/03/why-we-didnt-use-bloom-filter.html +[13]: http://redis.io/topics/data-types §"Strings" +[14]: http://redis.io/commands/pexpireat +[15]: http://redis.io/commands/evalsha +[16]: http://www.restmq.com/ +[17]: https://www.mediawiki.org/wiki/Redis