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Protocol Version 0
Mutable Version *.docx Immutable Version *.pdf
>> will clean this up tomorrow.
Abstract
The Hama Streaming Protocol is a protocol which enables non-Java applications to write BSP functions just like in pure Java code. The communication takes place with plain strings and is therefore highly portable. Also this is the only difference to Hama Pipes, which is a native implementation for C/C++ fully based on low level binary communications and is therefore faster and has less overhead. In the following chapters, we will introduce the text based protocol and its bidirectional communication, also with the practical example of a Python implementation. So we would be proud if other people take their time and implement the streaming protocol in other languages, for example in Scala or Ruby.
Streaming Basics
Hama Streaming is a special BSP implementation, which basically takes care of stream communication with the predefined protocol and by forking a child process with the given configuration. This configuration can be an interpreter, like python or bash, or a compiled binary. Hama now redirects the input and output streams of that child process to streams that are used for the protocol. Therefore you can’t simply print by writing to STDOUT, but you can use the LOG function that redirects special log statements to the log of the Java task, this will be explained later.
The protocol is designed to mimic the Java API, for obvious reasons of documentation and convenience, so we suggest to make yourself comfortable with the Java API first and then get into coding the protocol. For additional information about the design and programming model, have a look at our Getting Started section in our wiki and the user documentation as a PDF.
Getting Started - User Documentation as PDF
. http://wiki.apache.org/hama/GettingStarted#User_documentation_as_PDF If you want to know how to run a user code within other environments, have a look in the python example at the end.
Text Based Protocol
The text based protocol is very easy. You have a finite amount of operations that are associated with a unique identifier (OP_CODE). Common operations in BSP are sending a message or starting the barrier synchronization. In Hama Streaming, every operation is ended with a newline (‘\n’) character, so everything is done with a single line of text. Of course there are operations that are going to use multiple lines to naturally separate information, however these are special cases. In cases where the transferred information contains newline characters, you will run into problems, if you desperately need a fix for that, we want to add this in the next protocol version, if you can’t wait you need to prepare the transferred data that it does not contain newlines.
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Here is a table of all op codes and their corresponding unique identifier, note that not all of them are in use, because the streaming protocol is working on top of the binary pipes protocol.
Operation Code Name
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Operation Code identifier
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Comment
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START
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0
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First op code after fork
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SET_BSP_JOB_CONF
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1
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Get configuration values
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SET_INPUT_TYPES
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2
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Not used
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RUN_SETUP
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3
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Start of the setup function
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RUN_BSP
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4
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Start of the bsp function
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RUN_CLEANUP
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5
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Start of the cleanup function
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READ_KEYVALUE
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6
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Reads a key/value pair from input (Text Only)
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WRITE_KEYVALUE
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7
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Writes a key/value pair to output (Text Only)
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GET_MSG
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8
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Gets the next message in the queue
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GET_MSG_COUNT
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9
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Get how many messages are in the queue
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SEND_MSG
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10
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Send a message
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SYNC
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11
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Start the barrier synchronization
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GET_ALL_PEERNAME
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12
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Get all peer names
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GET_PEERNAME
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13
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Get the peer name of the current peer
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GET_PEER_INDEX
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14
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Get a peer name via index
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GET_PEER_COUNT
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15
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Get how many peers are there
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GET_SUPERSTEP_COUNT
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16
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Get the current Superstep counter
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REOPEN_INPUT
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17
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Reopens the input to reread
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CLEAR
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18
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Clears the messaging queue
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CLOSE
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19
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Closes the protocol
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ABORT
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20
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Not used
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DONE
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21
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Closes the protocol if task is done
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TASK_DONE
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22
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Yet another task is done op code
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REGISTER_COUNTER
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23
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Not used (please create a new issue for that)
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INCREMENT_COUNTER
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24
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Not used (please create a new issue for that)
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LOG
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25
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Not implemented in Pipes, but in streaming it sends child logging to the Java task.
Acknowledgements
To detect whether the forked child has arrived at the next stage of an algorithm, we work with acknowledgements (short ACK), that have a special formatting. ACKS are formally expressed as "%ACK_" + OP_CODE + "%=", where OP_CODE is the operation to acknowledge.
Initialization Sequence
After the child process has been forked from the Java BSP Task, a special initialization sequence is needed. Formally the sequence looks like this:
• START_OP_CODE from Java task
• Protocol number from Java task (currently actual version number is 0)
• SET_BSPJOB_CONF OP_CODE from Java task
• Number “N” of configuration items (note that this is a sum over number of keys and values)
• Now “N” lines follow, where the first line contains the configuration key, the following the value.
• Now the forked child can send an acknowledgement to the Java task
A sample communication could look like this // are comments and are not parts of the protocol:
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%0%=
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//
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start
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op
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code
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from
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java
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task
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0
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//
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protocol
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number
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%1%=
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//
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set
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bsp
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configuration
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op
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code
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2
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/
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number
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of
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configuration
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items
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to
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read
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hama.bsp.child.opts
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//
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sample
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key
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from
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configuration
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-Xmx512m
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//
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sample
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value
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from
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configuration
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Now the forked child needs to acknowledge the start OP code by writing:
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%ACK_0%=
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//
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ACK’d
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start
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code
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After the acknowledgement, we immediately start with the setup function. Setup/BSP/Cleanup Sequence
A normal BSP task has three major steps:
• Setup (used to allocate resources, save configuration values as variables, etc.)
• BSP (the main logic of your algorithm)
• Cleanup (free the resources claimed during the whole task)
The protocol is the same for every of the three steps:
• X_OP_CODE from Java task // where X can be SETUP (3), BSP (4) or CLEANUP (5)
• Now the forked child can run the bsp task of the user algorithm
• The forked child can send an acknowledgement to the Java task that the step has finished
Of course here is the example with the setup function:
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%3%=
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//
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setup
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op
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code
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from
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java
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task
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//
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now
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call
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the
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users
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setup
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function,
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any
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communication
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can
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happen
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here
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%ACK_3%=
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//
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ACK’d
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setup
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code
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The three steps are called in sequential order, so after you have ACK’d the end of the setup, the Java code will immediately start telling you about that you need to start the bsp function. This applies also for the transition between bsp and cleanup function.
BSPPeer Functionality
People familiar with the Hama BSP API know that there is a context object which gives access to the BSP functionality like send or sync. We think this is a cool design and you should take care of implementing it as well, so you can pass this peer in all the user implemented functions.
Here is the table for the communication of the currently implemented BSP functionality:
BSP Primitive
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Operation sequence
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Response
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Send
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- %10%=
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- Destination peer name
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- Message as text
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- Sync
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- - %11%=
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- After sync a special ACK will be in the next line: “%11%=_SUCCESS”
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So you should block until you received this.
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- getCurrentMessage
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- - %8%=
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- “%%-1%%” if no message was found, else the message as text
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- getSuperstepCount
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- - %16%=
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- The raw Superstep number
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- getMessageCount
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- - %9%=
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- The raw message count
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- getPeerName
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- - %13%=
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- Followed by an index (plain integer), -1 for the name of this peer.
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- The name of the peer as text
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- getPeerIndex
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- - %14%=
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- The raw index in the peer array
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- getAllPeerNames
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- - %12%=
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- A line how many peers are there (plain number), then n-lines with the peer names
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- getPeerCount
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- - %15%=
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- Plain number of how many peers are there
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- writeKeyValue
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- - %7%=
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- Key as line
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- Value as line
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- readKeyValue
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- - %6%=
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- Key and Value in two separate following lines. If no input available, both are equal to “%%-1%%”.
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- reopenInput
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- - %17%=
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Closing Sequences
To determine if the child process successfully finished its computations, we have a closing sequence that is expected after you acknowledged your cleanup. The closing sequence is basically just sending TASK_DONE and DONE command to the Java process. After receiving this, the Java process will do the normal cleanup and finish the task itself, after sending these codes you can gracefully shutdown your process by letting it exit with a zero status.
The end sequence looks like this:
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%22%=
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//
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task
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done
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%21%=
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//
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completely
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done
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Congratulation, you are now able to implement Hama streaming in other languages.
Appendix
Known implementations
Hama Streaming for Python: https://github.com/thomasjungblut/HamaStreaming
Running user code in the Python environment
In the Python runtime, you can pass a .py filename to the BSPRunner as argument, using import you can import the class, get the class via getattr and get an instance by instantiating this class.
Looks basically like that:
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| Wiki Markup |
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className = sys.argv\[1\] |
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module = |
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+import |
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+(className) |
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class_ = getattr(module, className) |
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bspInstance = class_() |
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Now you can pass this instance to the handler that calls back the user functions in the python implementation this is done by the peer itself.