Mercurial > urweb
comparison doc/manual.tex @ 1551:5175bed443f9
Warn about concurrency issues with message-passing
| author | Adam Chlipala <adam@chlipala.net> |
|---|---|
| date | Sun, 28 Aug 2011 14:43:30 -0400 |
| parents | 76dab73304a5 |
| children | d5c961c709f9 |
comparison
equal
deleted
inserted
replaced
| 1550:daf71c76707e | 1551:5175bed443f9 |
|---|---|
| 2031 | 2031 |
| 2032 The $\mt{channel}$ and $\mt{send}$ operations may only be executed on the server, and $\mt{recv}$ may only be executed on a client. Neither clients nor channels may be passed as arguments from clients to server-side functions, so persistent channels can only be maintained by storing them in the database and looking them up using the current client ID or some application-specific value as a key. | 2032 The $\mt{channel}$ and $\mt{send}$ operations may only be executed on the server, and $\mt{recv}$ may only be executed on a client. Neither clients nor channels may be passed as arguments from clients to server-side functions, so persistent channels can only be maintained by storing them in the database and looking them up using the current client ID or some application-specific value as a key. |
| 2033 | 2033 |
| 2034 Clients and channels live only as long as the web browser page views that they are associated with. When a user surfs away, his client and its channels will be garbage-collected, after that user is not heard from for the timeout period. Garbage collection deletes any database row that contains a client or channel directly. Any reference to one of these types inside an $\mt{option}$ is set to $\mt{None}$ instead. Both kinds of handling have the flavor of weak pointers, and that is a useful way to think about clients and channels in the database. | 2034 Clients and channels live only as long as the web browser page views that they are associated with. When a user surfs away, his client and its channels will be garbage-collected, after that user is not heard from for the timeout period. Garbage collection deletes any database row that contains a client or channel directly. Any reference to one of these types inside an $\mt{option}$ is set to $\mt{None}$ instead. Both kinds of handling have the flavor of weak pointers, and that is a useful way to think about clients and channels in the database. |
| 2035 | 2035 |
| 2036 \emph{Note}: Currently, there are known concurrency issues with multi-threaded applications that employ message-passing on top of database engines that don't support true serializable transactions. Postgres 9.1 is the only supported engine that does this properly. | |
| 2037 | |
| 2036 | 2038 |
| 2037 \section{Ur/Web Syntax Extensions} | 2039 \section{Ur/Web Syntax Extensions} |
| 2038 | 2040 |
| 2039 Ur/Web features some syntactic shorthands for building values using the functions from the last section. This section sketches the grammar of those extensions. We write spans of syntax inside brackets to indicate that they are optional. | 2041 Ur/Web features some syntactic shorthands for building values using the functions from the last section. This section sketches the grammar of those extensions. We write spans of syntax inside brackets to indicate that they are optional. |
| 2040 | 2042 |