SR_post

Sarracenia v02 Post Message Format/Protocol

Manual section:7
Date: March 2017
Version: 2.17.03a2
Manual group:MetPX-Sarracenia

SYNOPSIS

The format of file change announcements for sr_post.

A sr_post message consists of four parts: AMQP TOPIC, First Line, Rest of Message, AMQP HEADERS.

AMQP Topic: <version>.post.{<dir>.}*<filename>

<version> = "v02" the version of the protocol or format.
"post" = the type of message within the protocol.
<dir> = a sub-directory leading to the file (perhaps many directories deep)
<filename> = the name of the file on the server.

AMQP Headers: <series of key-value pairs>

"flow" = (optional) user defined tag.
"parts" = size and partitioning information.
"sum" = checksum algorithm and value.

Body: <first line> = <date stamp> <srcpath> <relpath> <newline>

<date stamp> - YYYYMMDDHHMMSS.ss - UTC date/timestamp.
<srcpath>    - root of the url to download.
<relpath>    - relative path perhaps catenated to <srcpath>
               may instead be a rename.

<rest of body is reserved for future use>

DESCRIPTION

Sources create messages in the sr_post format to announce file changes. Subscribers read the post to decide whether a download of the content being announced is warranted. This manual page completely describes the format of those messages. The messages are payloads for an Advanced Message Queuing Protocol (AMQP) message bus, but file data transport is separate, using more common protocols such as SFTP, HTTP, HTTPS, or FTP (or other?) Files are transported as pure byte streams, no metadata beyond the file contents is transported (permission bits, extended attributes, etc...) Permissions of files on the destination system are upto the receiver to decide.

With this method, AMQP messages provide a 'control plane' for data transfers. While each post message is essentially point to point, data pumps can be transitively linked together to make arbitrary networks. Each posting is consumed by the next hop in the chain. Each hop re-advertises (creates a new post for) the data for later hops. The posts flow in the same direction as the data. If consumers permit it, report messages (see sr_report(7)) also flow through the control path, but in the opposite direction, allowing sources to know the entire disposition of their files through a network.

The minimal layer over raw AMQP provides more complete file transfer functionality:

Source Filtering (use of AMQP TOPIC exchanges)
The messages make use of topic exchanges from AMQP, where topics are hierarchies meant to represent subjects of interest to a consumer. A consumer may upload the selection criteria to the broker so that only a small subset of postings are forwarded to the client. When there are many users interested in only small subsets of data, the savings in traffic are large.
Fingerprint Winnowing (use of the sum header)

Each product has a checksum and size intended to identify it uniquely, referred to as a fingerprint. If two files have the same fingerprint, they are considered equivalent. In cases where multiple sources of equivalent data are available but downstream consumers would prefer to receive single announcements of files, intermediate processes may elect to publish notifications of the first product with a given fingerprint, and ignore subsequent ones. Propagating only the first occurrence of a datum received downstream, based on its fingerprint, is termed: Fingerprint Winnowing.

Fingerprint Winnowing is the basis for a robust strategy for high availability: Setting up multiple sources for the same data, consumers accept announcements from all of them, but only forwarding the first one received downstream. In normal operation, one source may be faster than the others, and so the other sources' files are usually 'winnowed'. When one source disappears, the other sources' data is automatically selected, as the fingerprints are now fresh and used, until a faster source becomes available.

The advantage of this method for high availability is that no A/B decision is required. The time to switchover is zero. Other strategies are subject to considerable delays in making the decision to switchover, and pathologies one could summarize as flapping, and/or deadlocks.

Fingerprint Winnowing also permits mesh-like, or any to any networks, where one simply interconnects a node with others, and messages propagate. Their specific path through the network is not defined, but each participant will download each new datum from the first node that makes it available to them. Keeping the messages small and separate from data is optimal for this usage.

Partitioning (use of the parts Header)

In any store and forward data pumping network that transports entire files limits the maximum file size to the minimum available on any intervening node. To avoid defining a maximum file size, a segmentation standard is specified, allowing intervening nodes to hold only segments of the file, and forward them as they are received, rather than being forced to hold the entire file.

Partitioning also permits multiple streams to transfer portions of the file in parallel. Multiple streams can provide an effective optimization over long links.

AMQP TOPIC

In topic based AMQP exchanges, every message has a topic header. AMQP defines the '.' character as a hierarchical separator (like '' in a windows path name, or '/' on linux) there is also a pair of wildcards defined by the standard: '*' matches a single topic, '#' matches the rest of the topic string. To allow for changes in the message body in the future, topic trees begin with the version number of the protocol.

AMQP allows server side topic filtering using wildcards. Subscribers specify topics of interest (which correspond to directories on the server), allowing them to pare down the number of notifications sent from server to client.

The root of the topic tree is the version specifier: "v02". Next comes the message type specifier. These two fields define the protocol that is in use for the rest of the message. The message type for post messages is "post". After the fixed topic prefix, the remaining sub-topics are the path elements of the file on the web server. For example, if a file is placed on http://www.example.com/a/b/c/d/foo.txt, then the complete topic of the message will be: "v02.post.a.b.c.d.foo.txt" AMQP fields are limited to 255 characters, and the characters in the field are utf8 encoded, so actual length limit may be less than that. Sarracenia truncates header fields to ensure the messages can be sent.

THE FIRST LINE

the first line of a message contains all mandatory elements of an announcement. There is a series of white space separated fields:

<date stamp>: the date the posting was emitted. Format: YYYYMMDDHHMMSS. <decimalseconds>
Note: The datestamp is always in the UTC timezone.

<srcpath> -- the base URL used to retrieve the data.

The URL consumers will use to download the data. Example of a complete URL:

sftp://afsiext@cmcdataserver/data/NRPDS/outputs/NRPDS_HiRes_000.gif

Where the URL does not end with a path separator ('/'), the src path is taken to be the complete source of the file to retrieve.

Static URL: sftp://afsiext@cmcdataserver/

If the URL ends with a path separator ('/'), then the src URL is considered a prefix for the variable part of the retrieval URL.

<relativepath> the variable part of the URL, usually appended to srcpath.

<newline> signals the end of the first line of the message and is denoted by a single line feed character.

THE REST OF MESSAGE

Use of only the first line of the AMQP payload is currently defined. The rest of the payload body is reserved for future use.

AMQP HEADERS

In addition to the first line of the message containing all mandatory fields, optional elements are stored in AMQP headers (utf8 encoded key-value pairs limited to 255 bytes in length), included in messages when appropriate. Headers are a mandatory element included in later versions of the AMQP protocol.

flow=<flow>

A user defined string used to group data transfers together, unused by the protocol.
from_cluster=<cluster_name>
The from_cluster defines the name of the source cluster where the data was introduced into the network. The cluster name should be unique within all exchanging rabbitmq clusters. It is used to return the logs back to the cluster whenever its products are used.
link=<value of symbolic link>
when the sum is the 'link' header is created to contain the body of the symbolic link.

parts=<method>,<bsz>,<blktot>,<brem>,bno

A header indicating the method and parameters for partitioning applied for the file. Partitioning is used to send a single file as a collection of segments, rather than as a single entity. Partitioning is used to accellerate transfers of large data sets by using multiple streams, and/or to reduce storage use for extremely large files.

when transferring partitioned files, each partition is advertised and potentially transported independently across a data pumping network.

<method>

Indicates what partitioning method, if any, was used in transmission.

Method Description
p File is partitioned, individual part files are created.
i file is partitioned, but blocks are read from a single file, rather than parts.
1 file is in a single part (no partitioning)
  • analogous to rsync options: --inplace, --partial,

<blocksize in bytes>: bsz

The number of bytes in a block. When using method 1, the size of the block is the size of the file. Remaining fields only useful for partitioned files.

<blocks in total>: blktot the integer total number of blocks in the file (last block may be partial)

<remainder>: brem normally 0, on the last block, remaining bytes in the file to transfer.

-- if (fzb=1 and brem=0)
then bsz=fsz in bytes in bytes. -- entire files replaced. -- this is the same as rsync's --whole-file mode.

<block#>: bno 0 origin, the block number covered by this posting.

rename=<relpath>

The relative path from the current directory in which to place the file.

Two cases based on the end being a path separator or not.

case 1: NURP/GIF/

based on the current working directory of the downloading client, create a subdirectory called URP, and within that, a subdirectory called GIF will be created. The file name will be taken from the srcpath.

if the srcpath ends in pathsep, then the relpath here will be concatenated to the srcpath, forming the complete retrieval URL.

case 2: NRP/GIF/mine.gif

if the srcpath ends in pathsep, then the relpath will be concatenated to srcpath for form the complete retrieval URL.

if the src path does not end in pathsep, then the src URL is taken as complete, and the file is renamed on download according to the specification (in this case, mine.gif)

source=<sourceid>
a character field indicating the source of the data injected into the network. should be unique within a data pumping network. Usually is the same as the account used to authenticate to the broker.

sum=<method>,<value>

The sum is a signature computed to allow receivers to determine if they have already downloaded the partition from elsewhere.

<method> - character field indicating the checksum algorithm used.

Method Description
0 no checksums (unconditional copy.) Skips reading file (faster)
d checksum the entire data (MD-5 as per IETF RFC 1321)
L Linked: file is symbolic link, no checksum applies.
R Removed: file was removed, rather than updated, no checksum applies.
n checksum the file name (MD-5 as per IETF RFC 1321)
<name> checksum with a some other algorithm, named <name> <name> should be registered in the data pumping network. registered means that all downstream subscribers can obtain the algorithm to validate the checksum.

<value> The value is computed by applying the given method to the partition being transferred.

System Message: WARNING/2 (../sr_post.7.rst, line 314)

Block quote ends without a blank line; unexpected unindent.

for algorithms for which no value makes sense, a random integer is generated to support checksum based load balancing.

to_clusters=<cluster_name1,cluster_name2,...>
The to_clusters defines a list of destination clusters where the data should go into the network. Each name should be unique within all exchanging rabbitmq clusters. It is used to do the transit of the products and their notices through the exchanging clusters.

All other headers are reserved for future use. Headers which are unknown to a given client should be forwarded without modification.

EXAMPLE

Topic: v02.post.NRDPS.GIF.NRDPS_HiRes_000.gif
first line: 201506011357.345 sftp://afsiext@cmcdataserver/data/NRPDS/outputs/NRDPS_HiRes_000.gif NRDPS/GIF/
Headers: parts=p,457,1,0,0 sum=d,<md5sum> flow=exp13 source=ec_cmc

       - v02 - version of protocol
       - post - indicates the type of message
       - version and type together determine format of following topics and the message body.

       - blocksize is 457  (== file size)
       - block count is 1
       - remainder is 0.
       - block number is 0.
       - d - checksum was calculated on the body of the file.
       - flow is exp13
       - complete source URL specified (does not end in '/')
       - relative path specified for

       pull from:
               sftp://afsiext@cmcdataserver/data/NRPDS/outputs/NRDPS_HiRes_000.gif

       complete relative download path:
               NRDPS/GIF/NRDPS_HiRes_000.gif

               -- takes file name from srcpath.
               -- may be modified by validation process.

Another example

The post resulting from the following sr_watch command, noticing creation of the file 'foor':

sr_watch -s sftp://stanley@mysftpserver.com//data/shared/products/foo -pb amqp://broker.com

Here, sr_watch checks if the file /data/shared/products/foo is modified. When it happens, sr_watch reads the file /data/shared/products/foo and calculates its checksum. It then builds a post message, logs into broker.com as user 'guest' (default credentials) and sends the post to defaults vhost '/' and exchange 'sx_guest' (default exchange)

A subscriber can download the file /data/shared/products/foo by logging as user stanley on mysftpserver.com using the sftp protocol to broker.com assuming he has proper credentials.

The output of the command is as follows

Topic: v02.post.20150813.data.shared.products.foo
1st line of body: 20150813161959.854 sftp://stanley@mysftpserver.com/ /data/shared/products/foo
Headers: parts=1,256,1,0,0 sum=d,25d231ec0ae3c569ba27ab7a74dd72ce source=guest

Posts are published on AMQP topic exchanges, meaning every message has a topic header. The body consists of a time 20150813161959.854, a size in bytes 256, the number of block of that size 1, the remaining bytes 0, the current block 0, a flag d meaning the md5 checksum is performed on the data, the checksum 25d231ec0ae3c569ba27ab7a74dd72ce, a tag default and finally the source url of the product in the last 2 fields.

MetPX-Sarracenia

The Metpx project ( http://metpx.sf.net ) has a sub-project called Sarracenia which is intended as a testbed and reference implementation for this protocol. This implementation is licensed using the General Public License (Gnu GPL v2), and is thus free to use, and can be used to confirm interoperability with any other implementations that may arise. While Sarracenia itself is expected to be very usable in a variety of contexts, there is no intent for it to implement any features not described by this documentation.

This Manual page is intended to completely specify the format of messages and their intended meaning so that other producers and consumers of messages can be implemented.

AMQP Feature Selection

AMQP is a universal message passing protocol with many different options to support many different messaging patterns. MetPX-sarracenia specifies and uses a small subset of AMQP patterns. An important element of sarracenia development was to select from the many possibilities a small subset of methods are general and easily understood, in order to maximize potential for interoperability.

Specifying the use of a protocol alone may be insufficient to provide enough information for data exchange and interoperability. For example when exchanging data via FTP, a number of choices need to be made above and beyond the protocol.

  • authenticated or anonymous use?
  • how to signal that a file transfer has completed (permission bits? suffix? prefix?)
  • naming convention.
  • text or binary transfer.

Agreed conventions above and beyond simply FTP (IETF RFC 959) are needed. Similar to the use of FTP alone as a transfer protocol is insufficient to specify a complete data transfer procedure, use of AMQP, without more information, is incomplete. The intent of the conventions layered on top of AMQP is to be a minimum amount to achieve meaningful data exchange.

AMQP 1.0 standardizes the on the wire protocol, but leaves out many features of broker interaction. As the use of brokers is key to sarracenia┬┤s use of, was a fundamental element of earlier standards, and as the 1.0 standard is relatively controversial, this protocol assumes a pre 1.0 standard broker, as is provided by many free brokers, such as rabbitmq, often referred to as 0.8, but 0.9 and post 0.9 brokers are also likely to inter-operate well.

In AMQP, many different actors can define communication parameters. To create a clearer security model, sarracenia constrains AMQP: sr_post clients are not permitted to declare Exchanges. All clients are expected to use existing exchanges which have been declared by broker administrators. Client permissions are limited to creating queues for their own use, using agreed upon naming schemes. Queue for client: qc_<user>.????

Topic-based exchanges are used exclusively. AMQP supports many other types of exchanges, but sr_post have the topic sent in order to support server side filtering by using topic based filtering. The topics mirror the path of the files being announced, allowing straight-forward server-side filtering, to be augmented by client-side filtering on message reception.

The root of the topic tree is the version of the message payload. This allows single brokers to easily support multiple versions of the protocol at the same time during transitions. v02 is the third iteration of the protocol and existing servers routinely support previous versions simultaneously in this way. The second topic in the topic tree defines the type of message. at the time of writing: v02.post is the topic prefix for current post messages.

The AMQP messages contain announcements, no actual file data. AMQP is optimized for and assumes small messages. Keeping the messages small allows for maximum message throughtput and permits clients to use priority mechanisms based on transfer of data, rather than the announcements. Accomodating large messages would create many practical complications, and inevitably require the definition of a maximum file size to be included in the message itself, resulting in complexity to cover multiple cases.

sr_post is intended for use with arbitrarily large files, via segmentation and multi-streaming. blocks of large files are announced independently. and blocks can follow different paths between initial pump and final delivery. The protocol is unidirectional, in that there is no dialogue between publisher and subscriber. Each post is a stand-alone item that is one message in a stream, which on receipt may be spread over a number of nodes.

CHARACTER SET & ENCODING

All messages are expected to use the UNICODE character set (ISO 10646), represented by UTF-8 encoding (IETF RFC 3629.) URL encoding, as per IETF RFC 1738, is used to escape unsafe characters where appropriate.

FURTHER READING

http://metpx.sf.net - home page of metpx-sarracenia

http://rabbitmq.net - home page of the AMQP broker used to develop Sarracenia.

SEE ALSO

sr_report(7) - the format of report messages.

sr_report(1) - process report messages.

sr_post(1) - post announcemensts of specific files.

sr_sarra(1) - Subscribe, Acquire, and ReAdvertise tool.

sr_subscribe(1) - the download client.

sr_watch(1) - the directory watching daemon.

dd_subscribe(1) - the http-only download client.