I think he is talking about exactly that, and he said "locking", hilo doesn't need any locking max(id) lock all the table.
2011/2/2, Fabio Maulo <[email protected]>: > for MaxId do you mean > "select max(id) from table" ? > > On Wed, Feb 2, 2011 at 7:41 PM, Oskar Berggren > <[email protected]>wrote: > >> In other words, implementing such a "MaxId"-based generator should be >> perfectly doable, but it wouldn't be HiLo of course, and it could only >> use a table, not a sequence. >> >> /Oskar >> >> >> 2011/2/2 Jason Dentler <[email protected]>: >> > Here's my guess: All the hilo-style generators should use the same >> > algorithm. Internally, the SequenceHiLo generator uses a sequence to >> ensure >> > each sessionfactory gets its own next hi value. Therefore, the hi value >> must >> > increment by one. >> > Thanks, >> > Jason Dentler >> > On Wed, Feb 2, 2011 at 3:51 PM, Nick <[email protected]> wrote: >> >> >> >> As I understand it, the HiLo algorithm for ID generation essentially >> >> uses this formula: >> >> >> >> NextID = (Hi * Lo) + (++Counter) >> >> >> >> Hi is stored in the database, Lo is configured as part of the entity's >> >> mapping, and the Counter is just maintained in memory within the ID >> >> generator. Whenever Counter exceeds Lo (as new IDs are assigned to >> >> entities), a new Hi value is fetched, and Counter is reset. Also, all >> >> applications creating this entity basically have to use the same Lo >> >> value to avoid potential ID collisions, given how this formula >> >> evaluates. >> >> >> >> What I don't understand is the advantage this would have over simply >> >> storing a maximum ID as the Hi value, and incrementing it by the batch >> >> size you expect to use (instead of by 1). You would still need to >> >> fetch a new Hi value every time you run out of IDs, but there's no >> >> need to coordinate the batch sizes across applications, or across >> >> time. >> >> >> >> For example, if the current Hi value is 1, an application could lock >> >> and update that to 11 (batch size = 10), thereby safely ensuring it >> >> can safely assign IDs 1-10. Another application could lock and update >> >> it from 11 to 101 (batch size = 90), thereby ensuring that it can >> >> safely assign IDs 11-100. >> >> >> >> In either approach, locking of the Hi value in the database before >> >> updating it is crucial. But either approach would need to do the same >> >> number of database queries to fetch new Hi values (assuming the same >> >> batch sizes), and a "Max ID"-style approach has the additional >> >> flexibility of allowing the batch size to differ between applications, >> >> or over time. >> >> >> >> What is the advantage of HiLo here? I feel like I must be missing >> >> something obvious. >> >> >> >> -- >> >> You received this message because you are subscribed to the Google >> Groups >> >> "nhusers" group. >> >> To post to this group, send email to [email protected]. >> >> To unsubscribe from this group, send email to >> >> [email protected]<nhusers%[email protected]> >> . >> >> For more options, visit this group at >> >> http://groups.google.com/group/nhusers?hl=en. >> >> >> > >> > -- >> > You received this message because you are subscribed to the Google >> > Groups >> > "nhusers" group. >> > To post to this group, send email to [email protected]. >> > To unsubscribe from this group, send email to >> > [email protected]<nhusers%[email protected]> >> . >> > For more options, visit this group at >> > http://groups.google.com/group/nhusers?hl=en. >> > >> >> -- >> You received this message because you are subscribed to the Google Groups >> "nhusers" group. >> To post to this group, send email to [email protected]. >> To unsubscribe from this group, send email to >> [email protected]<nhusers%[email protected]> >> . >> For more options, visit this group at >> http://groups.google.com/group/nhusers?hl=en. >> >> > > > -- > Fabio Maulo > > -- > You received this message because you are subscribed to the Google Groups > "nhusers" group. > To post to this group, send email to [email protected]. > To unsubscribe from this group, send email to > [email protected]. > For more options, visit this group at > http://groups.google.com/group/nhusers?hl=en. > > -- Enviado desde mi dispositivo móvil -- You received this message because you are subscribed to the Google Groups "nhusers" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/nhusers?hl=en.
